I'd recommend learning how to cook basic, cheap meals before you go. Make a pot of (for example) chilli or curry. Divide it into 5. Freeze 4 of them. Rice costs next to nothing, and you've got five decent home-made meals for the few £ it cost to buy the ingredients. Pasta is also very cheap, as are potatoes.
Once you know what you can cook and what you need to buy, grocery shopping becomes incredibly easy. You can stock up on canned goods when they're on multibuy offers, share things like condiments with housemates, etc.