www.squaremeal.co.uk is good on listings...check it out for addresses and rough idea of prices, especially these:
Best inexpensive:
New Piccadilly, Denman St (original 50's cafe - good for lunch)
Busaba Eathai (various locations, eg Wardour St) - cheap but good Thai
Wagamamas - noodles and rice (various locs)
Best good but not THAT expensive:
Galvin on Baker St
Best expensive:
Pied a Terre- Charlotte St (excellent Michelin starred restaurant)
Tom Aikens - Chelsea
Hakkasan, Nobu
Pont de la Tour - good views of Tower Bridge, good food, but not as good as the others on this list..
All very subjective of course....!
RFPS