Leaving Heathrow for the afternoon after arriving from US

What number is “ many hours later?”

If this is the ‘typical’ 7 hour or so layover, remember some of it is taken up by going through security again after you get off the flight or when you return from the landside trip, plus the hour or so you might like to have to get to another terminal for boarding your next flight. I usually figure 2 hours is taken up just transiting through the airport. That 2 hours could easily be split for leaving and returning. Giving a bit of buffer time, might leave you with 4-4.5 hours off site.
Personally, I don’t know time frames for going through passport control/immigration to get landside or the reverse. Conveniently, there is a tube(train) line that goes into London from Heathrow. I’ve done it but don’t remember how long it takes. Runs frequently though and is easy to find at Terminal 5.
I sure would want to have the landside trip planned out extremely well, allowing for the ‘just-in-case’ portion of life.

And yes, if you’ve checked luggage, it will be checked through to your destination. What will you have as carryon or what might you need to haul with you for this jaunt? Might make a difference as to what you do also.

Are your flights on one ticket? Even if they are both BA they could still be two tickets.
Assuming everything is on one ticket, you will need to go through immigration to get into the UK and then on the way back go through security again.
The general advice is to arrive back at the airport at least 3 hours before, but given you will presumably have your boarding cards and the luggage will be forwarded to the second flight you can probably cut that a bit.
How long have you got between flights? If you’re thinking of going into central London you are going to need at least an hour each way on transport, plus say an hour to get out of the airport and say be back two hours before flight 2. So a 6 hour gap is already down to 1.
An alternative is to go to Windsor, maybe a 20 minute taxi ride away.

but I do think that our checked bags will continue on since the flights are all British Airways.

That is true IF the flights are on one ticket. Are they?

Thanks all. Our layover is 7-8 hours, and yes, it’s all on one ticket from British Airways. We’re considering going to Windsor for the 2 hour castle tour, and leaning more toward taxi rather than train for time constraints. Appreciate any feedback on transportation, or cost of taxi etc. Thanks again!

Realistically, arrival into the UK usually goes fairly quickly with the e-gates. Sometimes it is the amount of time it takes to get to passport control that can cause time to be extended. You might park at a remote stand and require a bus to the terminal. You might also need to take a train or long walk to passport control. I would allot an hour to be safe.

Although you will have checked your bags, I assume you will have a carryon that includes a change of clothes, medications, documents, etc. Think about sightseeing with that along. Will it work for you?

You need to check in at least 2 hours before your flight. The key here is to make sure that you don’t have any liquids on you, or you can pull your 311 bag quickly when you reach security. Electronic devices must also be removed. It is easy to forget if you are used to precheck.

Finally, keep an eye out for labor actions that might impact the airport. Our dear UK forum members are always very kind about posting strike notices.

You would have to do Windsor by taxi. The train is 2 changes in each direction, twice an hour taking 45 minutes overall, so far too risky.
There are two different bus routes- one takes 45 minutes, the other all but the hour.
So neither transit option gives you enough time in Windsor.
I personally wouldn't do it, personally I think it's too risky even by taxi, as you never know what will happen in either direction or in Windsor, but I am risk averse.
If you want to do it, taxi has to be the way to go.
Also if the TA flight was late or gets retimed at a late stage (or indeed your next flight retimed earlier, and your layover time was cut short), then if you had prebooked Castle tickets that money is gone.

If your flight arrives on time, then I think Windsor or even Central London is doable. Your biggest issue is going to be getting back to Terminal 5 at Heathrow for your next flight. Terminal 5 is very, very busy and you will need to be there at least 2.5 hours before departure even with no luggage to check. Security lines will often be snaked around and around. You can't predict what that line is going to look like.

That really only gives you 3 hours either in Windsor or London Central. You don't have to go into the castle, Windsor is quite lovely to walk around but book a private car to do Windsor. (public transportation will take too long). Or, if you want to just head into Central London, take the Heathrow Express from T5 (15 mins). Cheaper if booked way ahead. If you have never been to London before, I would head into the city. If on the day, your time is cut short, then you can stay at the airport and will have only lost the price of the HEX ticket. - I am also assuming you don't have any kind of mobility or health issues that would slow you down while getting around London. That could make a big difference.

Depending when your trip is you may need to factor in union strikes into your planning
https://community.ricksteves.com/travel-forum/england/strike-at-heathrow-terminal-5

You may want to consider getting a lounge day pass, taking a shower and just have a relaxing interlude vs. a rushed trip somewhere.

If you were going into Central London you wouldn't take HEX, which dumps you at Paddington, you would take the Elizabeth line which takes you straight in, and is just walk up and buy, not the ridiculous HEX buy months in advance.
Include the change at Paddington and the Elizabeth line takes no longer.
But a lounge at Heathrow is far more sensible.
I can't help contrast the advice on this thread with that on the 6 hours at Seatac thread, where the consensus of advice was to not go into the city, with one dissenting voice.
But either on HEX or Elizabeth line all you need to happen is for something to go wrong- a signal failure, train breakdown, electric lines failure, and you end up in deep trouble.

If you will be travelling before Easter then Terminal 5 might be a problem. That's where most BA flights are.

The security staff are striking from 31 March through Easter Sunday (10 days). I can't imagine how much fun going through security will be.

Fortunately after Easter. Can anyone give me a rough estimate of taxi cost one way to Windsor Castle??

And one more question….besides taxi cost estimate one way from Heathrow to Windsor Castle, will it be obvious how to get a return taxi from the castle to Heathrow, or will that be difficult? Thanks again!