We took my mother to London two years ago-she was 78- and we found that the tube was faster for getting around town- but not always easy because of stairs etc. Not all stations have escalators or elevators. The bus would enable you to see more of the city and maybe be a little easier to access for your mom- it will just take you a little longer to get from here to there. I wish we had done the bus because I felt as if we had my mother underground most of the time. The open bus tour is an excellent idea, and also getting some food from the Harrod's food court and having a picnic.
We also went to Greenwich- but it was quite a trip on the light rail- although they also have a leisurely boat trip up the Thames to get there. Then it was a long walk, and to get to the Maritime Museum and Prime Meridian line - where you can stand with one foot in the western hemisphere and one in the eastern- is quite a trek up a hill. We left my mom on a bench with some water because she didn't feel up to the climb.
We stayed in a small hotel, B & B in South Kensington, and I wasn't very happy with it. Also, it was near the time of the Queen's Golden Jubilee, and the taxi service that the hotel called for us to take us to Vicotoria Station tried to cheat us. He said that the roads to Victoria Station were all closed because of the Queen's jubilee, and he'd be happy to drive us to the airport for about $120. But luckily I had done my reading and when I confronted him with the fact that the Queen wasn't even in London that day, he backed off and we got to the station with no problem. So be careful.
I wish I had a hotel in London to recommend to you- but if no one on this chat line does, I would stick with some recommendations from a reputable guide book.