I live with my boyfriend, so £75 a week is our food shop. But he works full time so I'd say it balances out, and we ca afford luxuries so we have them. (Steaks, Ben & Jerry's, Tescos Finest and all those other bits and bobs that add up)
My biggest tip would be to buy spices/cupboard filler foods in bulk at the beginning of term (£20 will buy you one pot of eachessential spice, or buy a rack from Argos or whatever) and you'll be set, that way you only need stuff like tinned tomatos, chickpeas, black beans etc an you can make a lot of meals out of spices and tinned food.