It is possible to get something vaguely decent at Terminal 1. This is at Harry Ramsdens Fish and Chip restaurant. Not the best fish and chips Ive tasted, but better than everything else here (Pizza Hut, Garfunkels etc). Its a sit down, pretty quiet place and theres reasonable beer to wash it down with. Harry Ramsdens is in the main section *before* immigration.
At Terminal 4, where Ive had my share of moldy sandwiches, the best bet is Weatherspoons pub (after passport control). Weird choice I thought, but what's described as chicken balti was pretty good, and the sausages, beans and chips were fine.
(If you need a hotel, the Heathrow Hilton at T4 is convenient and its only a 6 min walk along a tunnel. And cant forget the Heathrow Express, which I hope most people know about. 15 min ride to London. Really, forget the tube.)
More on terminal 1
The flyertalk site that Anil Kular has mentioned offers tons of valuable info too.