I can reccomend the following as favourites of my dining companion & myself(in no particular order):

Le Gavroche (French/Mayfair/Mr Roux Jr), v.expensive but I have never eaten so much good food, my companion was equally as full. Bill (no alcohol) was about £180 for two with no prices on the menu handed to the ladies.

Matsuri (Japanese/St James) Good Tepanyaki cooking (i.e. on the hotplate in front of you) haven't sampled their other styles of cooking, cheaper than Le Gav, and for desert you must have Tepan Ice (fireball ice cream ^ ^ ^ ).

Babylon (High Street Kensington) This is a good place but let down by the owner being Sir Beard himself . The food isn't as good as say The Gavroche, but was still pretty good, and under £80 (again without alcohol being consumed). The setting is amazing as you have a fantastic view (ask for a table by the window) and are just above The Roof Gardens. If their isn't a function being hosted you could, (not sure as I haven't been for a while) go for a walk in the gardens.