Camden Town is a good place to go.

It's a food court of London, on the Tube's Northern Line, very easy to reach. Many restaurants are reasonably priced. Andy's Taverna, serving Greek food, is quite popular. You may have to book a table beforehand.

The area around Leicester Square, just nearby China Town and Soho, gathers many restaurants. Some of them are really good.

In addition, you may find good ones in Baker Street and London Waterloo.