London has become one of the world's great restaurant cities. The transformation since the 1990s has been real and significant: the old joke about English cooking is no longer applicable to London, which now has more Michelin-starred restaurants than Paris and a street food scene that draws on every food culture its immigrant population has brought.
Practically, this means you can eat extremely well if you know where to look, and extremely badly if you don't. The areas around major tourist sights — Leicester Square, Covent Garden's main streets, anywhere immediately adjacent to a museum — tend to serve mediocre food at high prices. Go one or two streets away and the picture changes.
For Indian food, Brick Lane in the East End is famous but increasingly touristy; better to head to Stoke Newington or Tooting for restaurants actually frequented by the Indian and Sri Lankan communities. Soho and Chinatown (around Newport Place) remain reliable for Chinese and Southeast Asian food. Borough Market on Southwark Street is a genuine food market, open Thursday to Saturday, and worth a visit for lunch on its own terms. Brixton Market is less polished but excellent for Afro-Caribbean and South American food.
Set lunch menus at serious restaurants are London's best-kept budget secret: places that charge £60–80 for dinner often serve two or three courses for £25–30 at midday.