Three of London's best restaurants:
Hawksmoor Seven Dials in Covent Garden (Langley Street)
St John in Smithfield (St John Street)
Mosimann's in Belgravia (West Halkin Street)
Three good restaurants (you could take a special date or a business client to any of these)
The Wolseley on Piccadilly
Cafe des Amis in Covent Garden (Hanover Place)
Bertorelli in Covent Garden (St Martin's Lane)
Five chain restaurants with lots of branches around London (fine for a regular meal out with friends or family)
Pizza Express
Cafe Rouge
Wagamama
Yo Sushi
Nando's
At the cheap-and-cheerful end, you have places like
McDonalds
Burger King
KFC
Subway
plus hundreds of fish and chip shops, kebab shops and Chinese and Indian takeaways.
PS: Never heard of Tim Horton's. =)