What you want are the places the Spanish dine in, most of them have something called 'Menu Del Dia' - menu of the day. You get a starter, a main course and a desert, and usually a soft drink or glass of wine, and bread, often for under five pounds a head. If the restaurant isn't advertising it - ask the waiter. They don't usually offer it to tourists as a matter of course, but they will if you ask.
Menu del Dia - meal of the day is about 10 Euros or �7. This is a daily menu which Spanish restaurants offered originally for working men but now tourist restaurants also offer this. Consists of starter, main course, pud and usually a bottle of wine between 2 people. Tappa bars where you can get a little of many of the dishes offered are 1 or 2 euros each and are very good for lunch.
Chinese restaurants are very cheap - the same type of 3 course menu from about 5 euros or main course only (larger portion) for the same price. Loads of English places doing usual brit stuff - all day breakfasts, burgers etc - at what prices they can get.
From there the sky is the limit - whatever you want to pay.