I'm in my last year of sixth form but I've been cooking for myself for a year and a half; exactly half of which I've been vegan.
It's cheaper than people think. The only real costs are when you want a meat or cheese substitute (think: Linda McCartney, Fry's, or Violife). You can buy vegan butter the same price as normal (or close to) with Vitalite. Aldi do ~65p soy or almond milk last time I checked, and tesco do cheap milks too. Tesco normally have Linda McCartney (especially the sausages) on some sort of offer and often half price (6 sausages for £1).
Don't be afraid to online shop, as you can often get good deals on the highest value items (3 for £6 on Fry's 500g Vegan Slicing Salami is one I'll never forget). They also normally have quite good deals (as Ocado do, in this example) on obscure things like frozen aubergine at just over a quid for a huge bag, fresh spinach on a 12x16 inch bag for just over a quid again, and so on. My girlfriend swears by going to a local market for her veg (Newcastle) and is also vegan; they do things like 4 avocados for £1, and cheap assorted fruit. Aside from this, I swear by tesco for my everyday shop in person as you can still get things for cheap if you spend the time to look around (belvita breakfast biscuits at £1.39 for 5 when you forget your packed meal, a good balance of cost and selection with things like mushrooms and sauces, etc). Aldi are cheaper but have a limited selection for vegans - if you're veggie, they'll probably be a much better choice though.
Tinned or bagged foods are the best bet for protein and other macronutrients. Peanut butter is your friend, don't be afraid of the cheapest bread you can find, and be prepared to drink a glass of soy milk as part of your meal. Also, take a multivitamin.
Examples of things I've cooked this past year have been ramen noodles (soy sauce, hot veggie broth, instant noodles, grated carrot, sliced onion/garlic/mushrooms. Almost certainly worked out at less than £1 per hefty serving, and then you can do the fun things like adding Miso paste on top of that while still having wiggle room for a tight budget. It's good to have base recipes prepared and be ready to spice them up as your finance allows it. Things like soups (boiled potato, frozen peas, onion and herbs) with toast make for meals at barely 50p a serving that will have your friends jealous of your bank account. It's much easier to be vegan than it is to be omnivorous in terms of cost at university, as long as you have the time to cook. Things like ready to wok noodles and a sauce packet with some grated veg will keep you full and fed with maybe 5 minutes of preparation and cooking time combined, while still tasting great.
Buy lots of herbs and seasoning before you study, they're your friends. Thyme, Rosemary, Oregano, Basil, Garlic Powder, Cumin, Chilli Flakes, Ginger, Smoked Paprika, and a couple of your favourites goes a long way. Use lots of soy sauce.