Food isn't cheap "out" around in the U.K, or in most of England, not when you're broke anyway. You can get some decent helpings of Chinese or South Asian food in parts of London though, for just a few quid.
Chips or Onion rings would probably be my favourite for "cheap" night snackage, with garlic mayo on them. Kebabs are crap here unless you're fucked off your tits, and burgers are £4 I can't afford (pizzas are even more), so....
Chocolate too, actually! Late night Snickers feasting hits the spot. Oh, the healthiness of Britain eh?

I love getting home and making epic peanut butter sandwiches actually....