I have stayed at a number of the Marriot properties in London.

If you are staying near the airport, the new LHR Marriot Slaugh (there is an older, farther LHR Marriot Slaugh, so be careful) is a quite nice business class style of a hotel.

For staying in the city, the Marble Arch is just ok. It is an older hotel, with small rooms, in an ok part of the city. It is within walking distance of Bond Street shopping and Hyde Park. An Underground station is just a few blocks away.

Grosvenor Square is a nicer hotel than Marble Arch. I stayed there on business a few months ago. Lets just say that if I had payed for the stay out of my own pocket, I would have been very disappointed. The rooms are small, and are could use a little bit of renovation. That said, the location is quite excellent. Right around the corner from the US embassy and other wealthy neighborhoods, excellent access to shopping on Bond Street, and just a few blocks away from an Underground station. Restaurants are small, and the lobby and public areas are pretty much non-existant.

The new London County Hall is in my opinion, a fantastic hotel. The location is just across the Thames River from Parliament, Big Ben and Westminister Abbey (try and attend the chior Sing Song on Wednesdays as the singing is beautiful). Trafalgar square and the National Museum (open late on Wednesdays) are within walking distance. I like to walk around when I visit cities, and so Buckingham Palace is also close by (approx 1.5 miles?) The view of the river from the restaurant is outstanding. There is an underground station just on the other side of the river. A nice day walk I like to take the underground to Leicester Square and the theater district, stroll through Picadilly Square and then by Buckingham Palace and the Pall Mall, through Trafalgar Square and then back along the Thames back to County Hall. (Maybe 5 miles?) There is a fantastic gym facilities, with an indoor lap pool (my wife say it and raved, I have to admit that I did not see the gym myself)
County Hall is also cheaper!

I may be in London in the beginning of December, and I will try to stay at County Hall.

Just my humble opinion