Your first year at Uni then?
See if you can befriend someone for meals and double your shopping budget then go buy in bulk. Buy things like Wholegrains; wholegrain rice, wholewheat pasta, potatos, try to only get bread the day you want to eat it at 9pm when its massivly reduced in the supermarket.
Proteins: lentils, chickpeas, beans, then look for frozen meat like chicken or even fish- some can be affordable if you only eat it once a week.
Veggies and fresh fruit- buy frozen as its cheaper, get big packets, store in the freezer take a little out to cook at eat. Get fresh stuff on the day from the reduced stock. Alot of lettuce gets reduced in the winter, alot of fresh fruit in the summer.
Milk and dairy are great when reduced too as they freeze well. Portion out into smaller containers (anything but glass!) before freezing.
If you haven't already got one, get your OWN mini fridge (I am assuming your flat/house sharing or in halls) labels cost money and usually get walked right over, if you have your own mini fridge in your room you can store your essentials and expensive things in there such as meat or heavily reduced stuff which would otherwise have costed a fortune.
If you are able to go halves with your room/flat/hall mates it may be slightly less risky to leave things in the fridge but at the end of the day, someone stealing your food could cause rifts as they are unlikely to replace anything.
Best time to hit the supermarket is an hour and a half before they close, locate the reductions aisle, then go do your shopping, by the time your done the reductions should be in place. Leave as soon as you have what you need- don't buy anything "because it was cheap" leave it for someone else who really wants/needs it, there is always someone whose favorite food your swiping 10 of 'just because it was 13p!'
Preplan your meals, precook and portion out large meals- bulk buy and store everything in the freezer. £40 a week is enough if you cook your meals in portions of 5-8 and freeze each portion (use a freezer bag) defrost each meal one at a time per evening in the microwave.
Staples?
Tinned tomatos,
Baked beans
Cheese- grate to make it go further, get medium chedder as you use less then mild chedder, add English Mustard to get a stronger cheesey taste.
Salt and Pepper
Vinegar- goes with everything!
Try not to add ketchup, its cheap but its sugary- will make you want to eat more then you actually need and so spend more then you actually need.
Jacket potatos
fresh seasonal fruit, try to keep up with whats in season as it will be cheaper. At the moment apples are good.
Good luck with your studies!