How about eating more vegetables and protein? You can get frozen fish in Aldi for relatively cheap (most people like cod or salmon

) Wrap it in tin foil and put dried dill (fresh dill would obviously be better but I'm thinking of a student budget

) and lemon juice on the fillet. Wrap the tin foil around and put in the oven. It should take about half an hour. Serve with baby potatoes, peas, carrots. You can buy mixed frozen vegetables, frozen carrots, frozen broccoli, sweetcorn in Tesco which are pretty cheap too. Why not try making your own sandwiches or soup or salad and bringing to lunch with you? If you buy lean ham, lettuce, tomato and skip the spread, it's pretty healthy.
Oh those tuna pasta box things are desperate! They just have loads of mayo in them, which of course bumps up the calories. Mayo is not your friend on a diet lol!!
Baked potato and beans..you can bake your potato in the microwave in two min, just put a cross on the potato at the top and then wrap it in kitchen paper (one sheet is okay).
Making your own salad is really nice-get slices of ham or chicken or turkey and put with lettuce, sweetcorn, beetroot, tomato, peppers, red onion. That sounds like a lot and it is but it's so low fat and will really fill you up
