Haven't seen it mentioned here, but I recommend the Halkin, in Belgravia. It's a LHW property, if I recall - small property, understated traditional exterior, modern-but-not-stark interior (corridors look a lot like those on the Death Star, actually), extremely comfortable. Spacious, very well-appointed rooms with extensive but subtle technology touches, and a Michelin-starred Thai restaurant. Quiet location near Belgrave Square, Green Park, Hyde Park, and (the service entrance to) Buckingham Palace.