It's been quite a long time since I've stayed in a hotel in London, but when I worked there, my (very fussy) US boss used to stay in the Knightsbridge hotel:
http://www.firmdale.com/index.php?page_id=16&sub_page_id=62
It's really lovely (all the hotels in the Firmdale chain IMO), with decent sized rooms, good locations, and impeccable service. Boutiquey-too. We used to have all our meetings there, often over a fab breakfast with room service.