Best way to get from Heathrow to our apartment and back again.

If you plan on taking the tube anyway, why not take it all the way from the airport? Piccadilly line (with space for luggage) to Hammersmith or Barons Court. Cross the platform (about 15 feet) and take the District Line to St. James. The fare, per person, from Heathrow to St. James using contactless--phone, credit card, Oyster Card--is £3.50 person. For four it's about $19 (thats USD)

For returning to the airport from Bristol, won't you have your rental car?

No we are planning on picking up the rental on our way to Bristol. I looked into renting a car in Bristol, but it is difficult finding a car that is big enough for all 5 of us.

Is everyone in your group old enough to manage their own luggage? Is all of the luggage "carry-on only"? Unless your answer to both questions is "yes," I would opt for the private car even if it is more expensive.

The Tube is definitely cheapest, but if you prefer to take the Heathrow Express (as we do), buy your tickets on the website more than 90 days ahead to get them for £5.50 each. Say by March 1 for your June trip.

I would then take a black cab from the stand right outside Paddington station to your flat, rather than transferring to the Tube and walking from St. James station. They can take four plus luggage—-we have done that several times. It is our “returning to London” treat.

Thanks everyone for your help. That gives me some good things to think about. Now how do I get back to the airport during rush hour traffic to pick up our rental?

https://tfl.gov.uk/\\

Useful tool for using public in London, seeking out routes and offering advice. But not for the train system outside the city area.

Thanks everyone for your help. That gives me some good things to think about. Now how do I get back to the airport during rush hour traffic to pick up our rental?

The same way you got to your apartment, you can do in reverse.

What time do you need to be at Heathrow to pick up your car?

If they need to be in Bristol by 11:30, they need to be at the airport rental car pickup a whole lot earlier than that. I won’t even try to estimate the driving time from Heathrow to Bristol.

But I will say that there is no way I would start my journey to Heathrow with a Tube ride during morning rush hour. The one time we attempted that, the trains were so packed we had to let two go by before we decided to just be more forceful and beg people to move back a bit to let us on—not easy within our luggage. We did figure out to go to the far end of the platform to board in the first car, which was a tad less crowded. But it was not a pretty sight, and I won’t try that again.

Now we generally spend our last night in London at a hotel a half-block from Paddington station so it is an easy ride to Heathrow in the morning for our flight home. If that means moving from our lodging elsewhere in London for that one night, so be it.

Europcar Bristol South is right next to Temple Meads station and does seem to have full-sized sedans, breaks and minivans suitable for 5 ppl and their (reasonable!) luggage. Whether prices are competitive vs Heathrow is another story, but worth another look perhaps? I'd happily pay a bit more NOT to drive the M4 from London to Bristol.

EDIT: Sixt also seems to be a reasonable distance away from the station, but Sixt is typically not the cheapest.

You might need to price this all out, but I agree with Balso. There are two trains an hour from London Paddington to Bristol and the journey takes 100 minutes. You would have a later departure from London, a more relaxed and shorter journey, and should be able to get the car you want as easily in Bristol as at Heathrow, and probably with a lot less hassle. As always, the devil can be in the details.
Have you driven in the U.K. before? The M4 from Heathrow takes possibly two and a half hours, and can either be an easy drive or a nightmare.

for those new to this party we had quite a lot of discussion, particularly in the return to London from Bristol a few weeks back.

That was at https://community.ricksteves.com/travel-forum/england/rental-car-and-departure-day-help and other threads.

For these specifics, if you are taking the Circle Line from Paddington all the way around to St James's Park I see no reason to pay extra and go the long way around using the Heathrow Express. Settle in to the Piccadilly Line tube train where it starts at Heathrow while there is loads of room on the train and you only have to enjoy the above ground scenery of suburban London on the way in before you go under ground as you near central London. One easy change to the District Line, cross platform, no steps, no walking, at Barons Court, then just 6 stops on the District Line and hop off at St James'. Upstairs and you're home. Just so easy.

If you want the Heathrow Express it is wait for the train which is less frequent than the tube, 15 minute ride to Paddington, find and walk to the Circle Line entrance and down stairs, make sure you get on it going in the correct direction (8 stops counter-clockwise, 19 stops clockwise) and then upstairs at St James's Park. 2 fares and no saving of time, and possibility of confusion.

You are arriving on a Monday (the Monday after the Queen's Jubilee Weekend) and leaving London on a Friday. Depending on arrival time of your flight and how quickly you are processed you will either arrive at the end of the rush hour or just after it.

Bristol is more fun. An hour to Heathrow, 30-60 minutes getting the car (plan for the worst and hope for the best) and 2 hours to drive, that's 4 hours so start out around 7:30?

I also would like to urge you to consider the train to Bristol. It will probably be a faster trip as well as more comfortable, and I would not mind paying extra for that.

I will also suggest the train to Bristol and pick up your car there. A lot less hassle.

Trying to get across London at that time of day at the end of your stay isn’t likely to be a pleasant experience with luggage. You would have to leave about 7.30 am. Taking the train would be less stressful.

Thank you everyone for your helpful suggestions. It has given me a lot to think about. I will have to look into everyone's suggestions.

Just one last question. Since we have to leave in the morning with luggage. Does it make more sense to grab a cab to Paddington Station or try to take the tub?

Thanks for all your help. Also thanks Southam for the link for the journey planner. That will be extremely useful.

So you will go to Paddington to catch the train to Bristol? That is the station the Bristol trains leave from. Well, I have already stated my opinion—-take a cab to Paddington, based on the very annoying and stressful experience we had trying to get on the Tube with luggage during morning rush hour. But I’ll bet most the responses you get here will be to just take the Tube.

you don't want to be herding your luggage into and onto the Tube at the height of the rush hour (albeit that Friday is often a little lighter and a little later, or was pre-covid), and your fellow passengers on the Circle Line won't like it either. Go for a cab. And demonstrate patience.

Can't see what the problem is in taking luggage on the tube is

It isn't luggage on the tube, it is St James's Park with luggage at peak time on the tube.

And five people with luggage on the tube at rush hour

Thanks, everyone. The more I research the more I realize traveling through England with 5 people is no easy feat. Thanks for helping me figure out the best way to do it so that everyone has a pleasant trip including me. Like Bob said the devil is in the details.

As long as you have reasonable amounts of luggage you can walk from St James Park to Paddington. It is a rather nice walk mostly through parks.