drop reltal car off at Heathrow or marble arch?

I would avoid driving in central London at all costs.

You don’t say which company you are hiring from, but I would look to see if they have an office in Basingstoke to drop the car, then take the train into London, which would be quicker than driving to Heathrow then having to get to South Ken. Taking public transport will be quicker than a taxi.

Turn the car in at the airport and take the tube (Piccadilly line) to the Gloucester or South Kensington tube stop, depending on which is closer to your hotel. It you drive into the city, there is a fee you’ll have to pay and London is very congested. The airport, especially coming from Bath, is easier, en route, and will be less stressful for you.

I most definitely recommend NOT returning your car in London itself. You don't want to pay the London Congestion Charge...and why hassle with London traffic?

Heathrow, no question.

Definitely do not drop the car at Marble Arch. Our first time ever driving in England we picked up the car at MA. We were headed to Bath and driving out of London was terrifying. LHR is easy to drop/pick up/drive out or return a car.

Heathrow- very easy coming off the nice big highway, and will have the longest operating hours. From there, there are multiple ways into London at different price points.

Which day of the week? What time of day?

Thank you,thank you, thank you, to everyone who chimed in. I guess we were just wishfully thinking and looking at a map that doesn’t show any cars, ha ha. I’m sure it would’ve been a disaster to go to marble arch.
Unfortunately we are traveling on a Sunday and have found out that the rental car agencies are closed on Sunday in most surrounding towns, therefore we have changed our reservation to drop our car at Heathrow . We will be arriving there around 19:00 from the west, probably taking the M4.
I am hopeful that there are cabs available at the Hertz Rent-A-Car location. I don’t want to have to go into the terminal with all my luggage to get a cab and or transport shuttle.
Thank you all again for taking time out of your day to help me out. Wonderful!😀🌵

I am glad you decided to drop off the car at Heathrow. I have rented a car at Heathrow many times, often through Europcar and, in 2017, from Hertz. I have never seen taxis at the rental cars' sites at Heathrow. There is an electronic gate to enter the sites...and one must check out at a booth upon exiting a different gate. The drivers of the shuttle buses to the terminals help with heavy luggage.

you might be able to get a lyft or uber from the rental car office, which would make things a lot easier. not sure what the rules are at heathrow, they may have strict rules about where they can pick up.