Washington, DC to Manchester or London -- visiting both cities

Go with option 2.
Do not book separate tickets.
You have to select Multicity on the British Airways or Iceland Air site site to book open jaw.
If you specify your travel dates someone could give you a recommendation.

Likely travel dates: Nov. 5 -- 14, approximately.

I would take an open jaw on BA. On the BA site, you need to select ”multi-city” to get that option.

My second choice would be your option 1b

(My personal preference would be to take United’s daytime flight (leaves IAD around 8am and arrives LHR around 9pm and start my trip in London. I much prefer the daytime flight to an overnight flight. Then I would take train to Manchester and fly home from there)

I found open jaw on your route costing $577; that is a price that I would take but the example below has a risky short a layover in Philadelphia

I would search both Expedia and British Airways and see if the prices are the same. Select Multicity before searching
Pick Washington all airports and it will include Baltimore and Dulles

Trip 1 BWI -MAN
Mon, Nov 5
7:11pm - 9:00am +1 Flight spans 1 day
8h 49m (1 stop)
Departure airport: BWI - 1h 11m in PHL Layover 1h 11m in PHL Arrival airport: - MAN
American Airlines 4861 operated by Piedmont Airlines As American Eagle, British Airways 1599 operated by American Airlines
Basic Economy

Trip 2, LHR - IAD
Wed, Nov 14
11:30am - 2:55pm
British Airways logo
British Airways
8h 25m (Nonstop)
Departure airport: LHR - Arrival airport: IAD
Basic Economy
$577

Look at multi-city option on BA webpage. You have to be on the computer to see it(not a phone).

That two hour layover does sound dicey though you are ensured to get a spot on another flight if you can't make the connection. I'd look to see if there was a slightly longer layover.

I don't do puddle-jumpers and wouldn't want a Philly connection, but the other options aren't so much better. Folks say transfers at Heathrow are not easy or quick.

Since I'm more concerned about delays going to Europe than on the return, I took a look at a reverse itinerary that would have you in London at the beginning of the trip and Manchester at the end. It means a shorter connection time at Heathrow--not ideal--but if you're willing to take the risk on the return...

Using multi-city on the British Airways website, I see 3 possible routes, each with a multitude of connection times offered.

One is the flight combination through Philadelphia that Jazz+Travels listed, all on American Airlines or their commuter partner. Various connection times at Philly available. The one I priced came to $695 round trip including the direct flight back from Heathrow. Overseas flight is on an Airbus 330. ( I always consider the plane type).

Second is a flight combo to Manchester through Dublin. The overseas flight is on "ASI for Aer Lingus" on a Boeing 757. Next flight operated by Aer Lingus. This came out at $775 RT with the same direct flight back.

Third is the connection at Heathrow, all on British Airways with a 747 for the overseas flight. This one is $755 for the dates I looked at. You can choose a long connection time if you are concerned, but remember that this will be on one ticket, so they will put you on another flight to Manchester if you miss your booked one due to late arrival of the overseas flight. So personally I would choose a 90-minute (or slightly more) connect time and not worry about it.

We prefer British Airways and almost always book with them. Heathrow is a huge airport but the flight connection path is well-signed. You will go through a different (easier) immigration path than the main one for entering and staying n London.

Just be advised that they charge extra to choose your seats if you want to do that in advance of the 24-hour check-in time.

I think United flies from Newark non-stop to Manchester, if that helps.

Option 1b. When you arrive on your direct flight (yes, avoid the hassle of changing planes) into London Heathrow, go to your hotel. If you are arriving early morning, consider booking it for the night before, too, so you will be sure the room is ready for you. Alert the hotel about this so they do not give the room away before your arrival! Spend a day or two relaxing and doing any sightseeing you feel up to doing. Then take the train to Manchester using Advance Fare tickets you have booked ahead online. Advance Fare tickets can only be used for the specific date and time you have chosen in advance. The trade-off, however, is lower fares.

Since you can't be sure how long you will need for passport control, luggage retrieval, etc. upon arrival at Heathrow, it would be difficult to book train tickets in advance. However, if you plan to stay over in London before taking the train to Manchester, you can choose the Advance Fare option for the day, time, and fare that you like best. I see on the National Rail website that most days in November the Advance Fare is 23 to 27 pounds, but if you purchase on the same day, the cheapest "Off Peak" is 43 pounds. You can purchase Advance Fare tickets now for your November dates. To have the best selection of fares, book as far in advance as possible.

Breaking up your London days may not be ideal, but should be more pleasant than other options.

If you are going to connect, choose the connection at Heathrow rather than Philadelphia. If for some reason you miss your connection, BA has multiple flights a day to Manchester and odds are you will arrive in Manchester the same day.

I know people complain about Heathrow, but I don’t think it is bad for connecting. I’ve missed connections twice at Heathrow, but voth times it was because my flight from the U.S. departed several hours late. Both times I got to my final destination just a few hours late.

Plan 1b. I'd take non stop flights over heming and hawing about splitting time in London any day.
Book flights immediately. Figure out how many days you'll spend in London before Manchester and get your advanced rail tickets for some cost savings.

Give yourself 2 overnights in London before taking the nice train jaunt to Manchester to visit your son.

You are on vacation, deep breathes, slow down. Spend time with son and either fly back to London or use the train again.

So much to see and do in London. Appears you'll be there for the Lords Mayor Parade on the 10th and Remembrance Day on the 11th. https://www.britishlegion.org.uk/community/calendar/remembrance-services-and-events/remembrance-sunday-2018/

Also check out Timeout London before you go. They'll have the times and locales for Christmas markets and lights. Same with chorale performances etc.

I travel to London nearly every November and am never at a loss for things to do and see.

Have a wonderful sojourn.

I think you'd be fine bookending the trip in London with Manchester in the middle. If you can swing your dates a bit and include Sunday nights in London, you'll likely find the hotel rates to be quite a bargain.

I agree with Laura on the daytime flight + train from London idea! I’ve also taken the daytime United flight on past trips. I think it seriously cuts down on jet lag even if you do stay awake a bit longer in your hotel the first night.

Hi C’ville, it looks like you live in Charlottesville, I love it there. My daughter has lived there since 2002.
If you go to aa.com then click on advanced search, then click on multi city. First flight type in Dulles Manchester, England, second flight would type in London Heathrow to Dulles. You will have to change plans in Philadelphia, then it’s a non stop flight to Manchester. I flew from a Philadelphia to Manchester non stop last September, flight was great.