There are two that I have stayed at that really stand out. The Grosvenor and the County Fair. They are walking distance from everything for me. Likewise, the Big Red Bus runs in front of both and is the way to enjoy London.
London, is special, Take a day trip to Paris on the Chunnel if you can.
We were in London this past June and stayed at the Renaissance. I can second spurfan's recommendation. It's a beautiful hotel, the service was outstanding, and it was very conveniently located. We did all of the typical 'touristy' things (Tower of London, London Eye, Westminster Abbey, Buckingham Palace, British Museum, Covent Garden, West End, etc) with ease of movement about the city.
I can highly recommend The Spa at Chancery Court, as well. I had an incredible experience!
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