rental auto in London

You didn't mention where to the north you are going, but I would recommend not renting a car in London. I would suggest taking the train north then getting the car. Like you said that a lot of rental agencies may not be open on Sunday so you may have to pick up a car on Monday. You could take the train to York (or another location), sight see there on Sunday and pick up the car Monday morning.

I would expect that rental facilities at Heathrow (at least) will be open on Sundays. may also be available at othr locations. they have tourists arriving 7 days a week. Easy to contact them and find out. I have never had a problem in large towns

I checked a couple of agencies in York and they are closed on Sunday, I think most of them are.

You can return a car to Heathrow for a reasonable drop-off fee if you rent it in London. (And the airport agencies will be open on Sunday.) If you'e flying home from Heathrow you won't want the hassle of returning the car to London and then making your way out to the airport by other transportation. Picking up a car in London and driving north is challenging but doable. I picked up a car near Kings Cross station and it was difficult maneuvering with all the aggressive buses and taxis. Returning it near Marble Arch on a Saturday morning was easy, though. Driving in and out of the car rental agencies at Heathrow is relatively easy, too, and you are well-placed from there to head north. Only you can decide the better option for yourself: Is the convenience of picking up the car in London worth a half-hour to an hour of very stressful driving? If you're not a good and confident driver, I'd suggest going out to Heathrow to pick up the car. If you're up for the challenge to leave from London, have a very good map and a navigator in the passenger seat! Good luck and have fun!

Assuming a decent deal by picking up in London compared to an airport, I would go with the city location. As Andrea mentioned, some agencies are open and Sunday traffic won't be bad. With five people and luggage, it would be easier and probably quicker if a couple of you get the car then return for the others and the bags, then head out of town. In addition to a good London map and a navigator, I recommend a GPS. A problem I've consistently encountered in London driving is finding street signs. Sometimes they're up on the side of a building or down on a low garden wall, not always facing the right direction for you, and sometimes they're non-existent. DW and I have missed many a turn through the years, but GPS has fixed that. Driving in London is a lot easier with it. GPS, called SatNav in the UK, will also be useful on your trip, so if you don't have one with European maps installed, get one and practice using it before the trip. For maps I recommend a color London A-Z and the AA large-scale Atlas of Great Britain, both of which you can get in any bookstore in London.

Re-reading your post, I think I misunderstood your Sunday issue and thought you would be returning on a Sunday rather than leaving. Some London agencies are open on Sunday -- Alamo near Victoria Station, for example. If the agency you use has an office in London open on Sunday, personally I would pick the car up there and return it to Heathrow. Sunday traffic should be fine. (Still advise the map and someone who can read it, though!)

Good tip by Tex about the GPS. I've also found that the larger gas stations carry a great selection of maps and atlases, and they also have the local maps as you trave through. I like the large spiral-bound AA or Collins UK atlases that can be folded open on someone's lap.