I asked a similar question about a year ago and got some good suggestions (similar to what is posted here)...and we ended up staying at The Capital which we were very happy with (booked by FTer DavidO, a Virtuoso agent) -- a small, more boutique-like hotel, just down the street from Harrod's, where they called us by name the whole weekend, we had wonderful meals, and the room was quite comfortable.

I think location is an important factor in London -- will you be walking around, and if so, where? do you want to be near a tube stop? etc. So advise taking that into consideration as well, since there are so many fine hotels to choose from.