How far in advance to buy tickets from US to London?

No doubt that price will fluctuate between now and then. I assume you are flying American airlines so sign up for a sales alert with them. Otherwise the only way to shave several hundred dollars off that fare if you want to buy now is select one with connections

No way to know for sure (frustratingly).

Last year I bought a nonstop ATL-LHR ticket on Delta during the last week of December for June travel. First week of January they had a sale and the fare for the same flights was 40% lower!

Delta — to their great credit — gave me the entire difference as a voucher (i.e., waived the change fee) because I’d bought the ticket less than a week earlier and, let’s be honest, because the agent was nice. Medallion status on Delta also helped I’m sure.

Good luck!

If you want a non-stop flight your only choice is American Airlines, and using the ITA Matrix monthly view, one can see that the price is pretty much $1800 every June departure day, except June2 which is $1619.

If you want a lower price, accept one stop on the way. Air Canada is as low as $1245, again according to ITA.

No one knows if the price will go up or down. The one time I tracked fares, I watched British Airways flights from December up until May for a July trip. For Seattle to London it started around $1400, jumped quickly to $1600, then rose slowly but steadily to $1800. Never dropped during that period.

But that is just one airline, 5 years ago. Prices fluctuate depending on the competition, weather, political events, the US economy, and all kinds of other factors that affect demand. The best advice is to watch prices daily, decide what is reasonable for you, and buy if and when you see that price. And don't look back.

When comparing fares, make sure you compare apples with apples. So fares shown are basic Economy fares where you will pay extra for seat selection, checked bags, in flight entertainment, even meals. Also look carefully at the route if it is a flight with one stop. The layover could be hours and add considerably to the overall length of your trip.

It might be because your originating airport is Charlotte. We got O'Hare to London for $1200. If your willing to do stops it might get cheaper. We bought ours about 10 months out from our trip.

June is prime tourist season. I fly out of Raleigh, but check Charlotte prices, and while $1800 seems high, I've never seen prices much below $1200.

Check prices flying out of one of the major east coast airports: Dulles, the NYC airports, Boston Logan, etc. The cost of getting there and back from Charlotte plus the London flight might be a savings.

If you run a Google search -- CLT-LHR or Charlotte-Heathrow -- you get an option to sign up for email alerts when the prices changes.

Bargain airlines like Norwegian are considerably cheaper but you need to know what you are buying. The cheapest ticket gets you on the plane, but typically everything else, from a seat reservation to carry on to checked luggage adds considerable cost. Some of the majors also have similarly priced cheap tickets, so you need to read the details.

Wow what a difference a month makes! We are flying May 9-June 1st and I got 2 tickets on British Airways for $1264!
Non stop Denver-Heathrow.

Your budget option would be to fly to either Boston or New York JFK on Jet Blue. At either place, you would take a connecting flight to London Gatwick on Norwegian Air Shuttle.
Sometimes Jet Blue is inexpensive and sometimes it's not.

We are flying 5/25/2019 Nashville to Boston on Delta and connecting into Madrid on Norwegian Air Shuttle.
Our return flight is Paris de Gaulle to Boston on 6/9 and Delta to Nashville on 6/10. We've got to spend the night in Boston at a LaQuinta for $117. The total airfare is $700 round trip which is $1000 cheaper than the big U.S. legacy air carriers are charging on the same route. Boston-Madrid is a new flight for Norwegian, and often new flights get extra cheap airfare for the first month or two.

No one knows what will happen with airline prices. However, in watching from my (completely different, more-competitive) origin airport, I have gotten the impression that the gap between non-stop fares and 1-stop fares tends to widen as you get closer to departure time. If you are adamant about a non-stop I would be cautious about expecting/hoping to snag a deal in March or April.

On the other hand Brexit is a new factor. If things go badly in a public way, who knows how many people might decide to go somewhere other than the UK next June?

Are you interested in visiting Dublin? Non-stop flights from Charlotte to Dublin look to be around $1000. There are many inexpensive flights (<$100) from Dublin to London.

I fly L.A. to London in June frequently. I've never paid nearly that much.
If I can't find a fare that I like from here, I try other major cities that are an easy flight from here. Last year I found a fare from San Francisco, one way, for under $400. So I used my Jet Blue points to fly to San Francisco (cost me $5 in taxes) and then flew from there. Found another one way home for $545.
If you are absolutely set on non-stop you'll probably not do much better than $1800 direct from Charlotte...in fact, I think that's probably quite reasonable for a direct flight from there. Unfortunately, the cheaper fares aren't available from all airports...usually only major hubs. Charlotte is a big domestic hub but not as much international. My knowledge base says that either New York or Boston will be the best bet from the east coast.
Update...just for fun I looked on the Norwegian Airlines website at their low fare calendar for June and here the result. If you are willing to fly from Boston you will save a bundle! Here's the link...I fly Norwegian to London and other places frequently and am perfectly happy with them. Even with baggage fees and choosing your seat (there are charges for both) you will still be WAY ahead. Bring your own food...theirs is terrible. Everything else is great! Comfortable seats, fantastic entertainment system.
Now that I've looked I'm itching to buy a ticket!!

I've flown out of Boston several times because BA has an early morning flight to Heathrow. I've noticed BA often offers some very cheap everything-is-extra prices on that flight.

(If you are tall like me and want to upgrade to get more legroom, check all of BA's fees for doing that. The equivalent upgrade may be less expensive on American or one of the other airlines selling codeshare tickets on the flight.)

A downside of flying out of another city happens when your schedule means you need to spend a night there.

If you really want that non-stop flight, there is a way to get one ticket for much less, although you are running short of time to do this. Here's how:

Join the American Airlines Miles program and get their credit card. Spend $2000 on the card right away and you will have 42K Miles. Buy 18000 more for around $500, and you will have the 60K Miles needed for an Economy ticket. There are still dates in June with Saver awards (30K each way) on the direct flight from Charlotte to London. You will have to pay full price for the other ticket, but the total for both will be around $2400, instead of $3600.

The downside of this plan is that you two will be on separate tickets, which can be a pain if there are schedule changes. But with non-stop flights this is not such a big deal.

The other problem is those Saver awards on the direct flight could disappear in the time it takes to get the credit card and earn the first 42K Miles. So it is a gamble. But there would still be awards on one-stop flights, so you could still benefit on the price.

As others have said, your best bet for a cheaper flight overall would be to fly from Charlotte to one of the major east coast airports (NYC, DC, Boston) and then fly to London from there. We’re flying from JFK-LHR on August 16th and I got my tickets already, $575 per person round trip on American (operated by British Airways) in Main Cabin Economy.

A couple things to be aware of:
If you set up a price alert on Google Flights, it will automatically show you Basic Ecomony fares as the lowest price on airlines that offer it (of which American is one).

If you do fly through another airport and end up on British Airways, they charge extra to choose your seats in advance no matter what class of ticket you buy. I booked my tickets through a 3rd party and didn’t realize this when I booked the BA flights. (I decided not to pay the extra; since we’re flying with children BA will assign us seats a week prior to departure and guarantees at minimum that each child will be seated with at least one adult from the party.)

Sign up for British Airways e-mail newsletter, you'll then be advised of when their sales are on (they have several throughout the year). Be quick because Virgin Atlantic have just released their sale so I suspect BA will not be far behind. There's no rhyme nor reason when they release their sales but I'll put money on one being very soon.

Flying from CLT is totally different from what I do. In my experience (competitive market), airfares gyrate pretty constantly, irrespective of whether there is some sort of a sale supposedly going on. Now, that may not be the case for you, but I would not depend on a sale notification if I were looking for an opportunity to save money on my airfare. I would be checking fares every single day myself.

I would check fares every day, and be prepared to buy immediately when you see a price that is good.
Prices fluctuate every day. As it gets closer to departure date, airlines want to fill up seats, so you may get a better price.
Keep checking. You don't have to buy now. But when you see the price you want, jump on it within 5 minutes.

I follow different cheap airfare pages on Facebook and get alerted to random good deals. We scored tickets for $700 each for our upcoming trips. Airfare to Manchester is on sale currently out of some US cities for around $400.

In 2015 I paid close to $1800 for tickets to LHR from St. Louis and I see the prices are up there again for next summer. Over the past couple of years, I have brought separate tickets from Chicago to LHR on AA for around $700 - $800 and then brought a $280 dollar round trip ticket from STL to ORD. I see you could get similar prices out of Charlotte buying tickets on a separate itinerary on AA. So you could potentially get to London for about $1000 for June on AA. I would strongly suggest you give yourself a huge lay over time if you do this - you cannot take a chance on missing the ORD to LHR flight (Or what ever East Coast hub you find a deal from). I even fly in the day before and get a hotel. If you are buying a couple of tickets that is a potential saving of $2,000. I would select a couple of potential routes and check everyday. weigh up the pros and cons of spending a night in a lay over airport hotel. If you have the time - its worth it to save that kind of money.

irrespective of whether there is some sort of a sale supposedly going on

The sales with BA are genuine. We've often got prices for half of what they are usually however they are typically business class so the savings are more obvious.

A BA sale started today, but good luck trying to book. Apparently the website and call centers are overwhelmed with the Gatwick airport mess.

Maybe because it's June when you want to travel, but $1,800 would be a no go for us. True we fly out of JFK, but we'll be flying roundtrip to London for $575 including seat selection and one checked bag at the end of March. Charlotte is an AA hub so they and their partners like BA really take advantage of that.

Carol, we (2) just traveled to London (October) and we flew 1 way non-stop to London via Delta for 1K and that was comfort class for $525.00 per person. I believe main cabin cost was about $3XXxx. Since it was a long flight 6 hours thought we would try comfort class. And we purchased our airfare about 4 weeks before departure. Wish you luck and enjoy your trip.

We booked in March (leaving NJ tomorrow for London, staying thru New Years Day) and paid $525 per person, non-stop. I literally started checking the flights every day last January and vowed to pull the trigger as soon as they dropped below $550 -- no matter what. If you're patient and willing to roll the dice, I'd wait.

I second the person who mentioned Dublin. We flew from Portland to Chicago (4.5 hours) and then to Dublin (7.5 hours) for $850pp in August of 2018. I booked those tickets in late fall of 2017. (Briefly in spring of 2018 the same route dropped to around $700.)

We were willing to have the stops because it gave the kids some wiggle room and overall flight time was only 12 hours. Ryan Air is cheap out of Dublin to the UK. We booked with United and the flight was fine and we got 3 rows to ourselves for 4 people on the way east. It left Chicago at 9 p.m.. On the way home, United changed our flights and due to a work project they switched us to Delta. Flights were longer, but plane was nicer, but it was packed and travel schedule took 24 hours. That said, no baggage fees, free movies, and food (more on Delta).

I highly suggest subscribing to Scott's Cheap Flights. A few days ago, they sent an email with price drop info for what I was looking for. I got direct flights, Austin to London Heathrow for $850 each.

Unfortunately, you're at the mercy of an airport without any healthy competition among carriers, so that severely curtails your options. Charlotte Airport is dominated by American Airlines and has virtually no rivals, including budget carriers (this was covered in a good article in the Wall Street Journal a few months back) - that's why you're seeing these really high fares (no way would I pay that much to go to Europe from the East Coast). I would try not to fly out of Charlotte if possible and use it as a jump-board to another airport with better fares. There is really no incentive for AA to lower fares for peak travel season given their market share in Charlotte.

See if you can access article:
https://www.wsj.com/articles/why-an-american-airlines-monopoly-works-for-charlotte-1519225740
"CHARLOTTE, N.C.—Many cities with airports dominated by one big airline have found they can bring in growth and low fares by luring low-cost competitors. But that’s not happening here.
American Airlines carries more than 90% of all passengers at the airport....."

I watched our preferred flights fluctuate $600 to $1,000 in price for two tickets, so we finally picked a number and purchased on November 24. We bought open jaw and booked everything through BA.

BA premium economy June 20 from New Orleans non-stop to London;
BA economy June 26 from London to Milan;
AA premium economy July 14 from Venice to Chicago non-stop; and
continuing through AA economy from Chicago to New Orleans.

Including a $120 fee to select seats, plus other fees and taxes the total for two was $4,340. Last year we booked transatlantic through AA economy and paid $3,400 from New Orleans to Charlotte, Charlotte to Paris, Paris to Rome (Air France), Rome to Charlotte, Charlotte to New Orleans. Flights from major airports are less, but the expense and time to travel to them outweighed the savings for us. I hope our experience is helpful.

Agnes's point is important (unfortunately, I can't access her linked Wall Street Journal article since I'm not a subscriber, but you get the point). Flying out of Charlotte is quite different from Boston, New York, Los Angeles, DC, or any of the other places others have mentioned as having lower fares. Those places have lots of competition on nonstop flights to London, and even more options with one stop flights via Keflavik or Dublin. This keeps fares down.

With Charlotte having no competition, AA can keep the fares high - and that's what they've done. From Wikipedia, I learned the term for this situation - "fortress hub": https://en.wikipedia.org/wiki/Airline_hub#Fortress_hub

You certainly can keep checking, but don't get your hopes up for a much lower fare on this nonstop flight. Your options are to pay what AA is charging, or to do a one-stop flight, or to book separate tickets to a cheaper place like Boston. If you do this, you will probably need a night in a hotel to make sure you don't miss the transatlantic flight, but you can probably still save money (at the expense of time - isn't that always the trade off?).