Heathrow Timing - Advice

because it will be a brand new flight, we will have to go through customs

Common misconception, customs takes zero time (actually about 30 seconds as you walk through the "nothing to declare" door).
What takes time, and has a high risk of added delays, is immigration (passport control). This could take typically 10-60 minutes.

, go get our luggage and go BACK through security

But that is not all:

• Taxiing, disembarking, walking through corridors = 30-45 minutes
• Immigration = 10-60 minutes.
• Luggage reclaim = 5-10 minutes (all the time you spent in immigration gave them time to get your bags to the carousel)
• Customs = 30 seconds
• Getting to check-in for 2nd flight = 10 minutes if same terminal, 30-60 if different terminal.
• Check-in for second = 1 hour minimum.

Conclusion: allow four hours minimum.
But, even then it can go pear-shaped if your incoming flight is late or immigration really backs up.
And, if you miss your second flight you will have to buy a new ticket.

11:00 + 4 hours = 13:00. + 1 hour flight + 1 hour time difference = land CDG 15:00. Same procedure at CDG (Immigration etc.). Get to your hotel ~18:00, just in time for dinner.
Personally, I would overnight in London, somewhere near Kings Cross and get an early Eurostar to Paris the following morning and arrive relaxed.

Another issue is that we're trying to avoid the one random overnight since we have 9 days at the back-end of our trip back in London. We are hoping to do the 3 days in Paris at the front end so that we can rent an AirBNB flat for 9 consecutive days and not have to hop around too much.

But your advice helps a bit... there is a 16:00 flight into Orly which we were considering, but you're right; it is a LOT in one day. We're actually leaving Charlotte Sunday with a layover in Toronto, so we are talking 24 hours of airplanes/airports and I suppose we are trying to decide if we want to just get all the air travel out of the way at once.

I know you all think we're buffoons. :) We planned on a week in London and a week in Barcelona and recently decided to scrap Barcelona in favor of (most of) the full time in England, so we have had to scramble a bit.

Here's another option for the day we land at Heathrow.

Stay in Salisbury, knock out going to see Stonehenge and then go to Paris the next day....?

What about putting your 9 days in London at the front end of your trip and the 3 or so days in Paris at the end? Then you could either take the Eurostar or a flight without the worry about delays, etc.

There's actually no good reason why that's not an option....

I will look into that! Not sure why that didn't cross my mind in the first place. I'm a little overwhelmed by all the last minute changes.

I'll second what Toni said - why not put London on the front end and take Eurostar? Honestly, Euostar is quite easy. You can certainly add a flight, but you'll want to have PLENTY of time. Further, if you fly to Paris, you then have to navigate back into the city (quite doable, but if you havent done it before, it can be a challenge, esepcially as you will likely be exhausted by the time you get there.) Taking the Eurostar lets you out at Gare du Nord, in the heart of Paris.

If you decide to fly, I'd allocate a bare minimum of 4 hrs to get you through immigration, collect your bags, and get back through security.

If you decide to Eurostar, just book a later train to give yourself a chance to actually breathe after flying, get a bite to eat, and do things at a slower pace. You are going to have to clear immigration anyway, so why not just go into town and catch the train?

Last question hopefully... if we add Paris at the end, Eurostar round trip and then take the Heathrow Express straight to the airport?

Or Eurostar to Paris and then fly into Heathrow from CDG or Orly to head home?

As they say "six of one, half a dozen of the other.
Do you want your city centre to airport leg to be in Paris or London?

I would pick Eurostar, as it is more comfortable (I hate flying), and more reliable on time. And you don't have to !*#§ around with check-in and luggage reclaim.
But, it is personal preference.
You can get the Piccadilly Line direct from St Pancras to Heathrow, Takes 1 hour and is a bit packed. The alternative is circle line to Paddington and Heathrow Express, a few minutes quicker but more luggage hauling when you change at Paddington.

If its me, I Eurostar r/t, and pick my "city center to plane" leg to be London. I just find London easier to navigate personally, though either is eminently doable.

I am also of the "I prefer the train" crowd. Its just more comfortable. Circle or Hammersmith&City tube lines get you Kings Cross to Paddington, and Heathrow Express from there. You can buy Heathrow Express tickets in advance too, to cover your round trip. Its a little quicker than taking the Picadilly Line, but a bit more faffing with luggage.

As Chris says, Picadilly Line will also get you direct to Heathrow from Kings Cross in about an hour. Its direct and easy, though a little more crowded. You likely wont be alone traveling with luggage.

The third option is of course to come off of the Eurostar, and get a black cab to Paddington for the Heathrow Express to LHR.

Super helpful, as we've never been to either London or Paris so I can't really say which city center I would prefer.

But like you two, I also hate the airport and planes and would much rather be on the train, so it sounds like r/t Eurostar will be our best bet.

Thanks again, I have a lot to do this evening -- really appreciate all the advice!

Another option is to stay 1 night in Windsor when you arrive in London. It's a quiet little town where you can get over your jetlag your first day. It's about a £16 can ride from Heathrow. Then the next day, take a cab to Heathrow (terminal 5 is best in this case) so you can can catch the Picadilly line to St Pancras and then take the Eurostar to Paris. This way, you're well rested and not rushing to get from one place to the other. Arriving around 11 am, you will have time to see Windsor Castle and have a nice dinner before going to sleep! We stayed at a great B&B called 76 Duke Street in Windsor.

Have you bought your transatlantic airplane tickets? You have only given us your departure date but not the return date or airline. Which is not a big deal. If you have not look into a multi stop ticket ...into London out of Paris.

If you have your airline tickets another suggestion would be: You could look into paying the ticket change fee and return from Paris. If you consider the cost of the return Eurostar tickets and perhaps another night of London area lodging it might not be a huge price difference and another solution.
Just do the math and consider the convience....

Yes, we had already bought our tickets months ago in and out of LHR and got a great deal. I did check into changing the flight to Paris, but since we got the transatlantic tickets at a steal, the fare difference plus the change fee would cost us over $1k, so that one isn't really an option.

I have to thank you all again for your help... we moved Paris to the back end of our trip and by some miracle, I checked United tonight again to see about flying home to Charlotte from CDG instead of Heathrow and there was NO fare difference, only the change fee!!! So with the time savings, one way Eurostsr savings and just overall less headache, we decided to pay the change fee and are now flying home from Paris.

I appreciate you all listening and suggesting! This forum has been invaluable to me.