It should be fairly easy.
Rice, pasta, lentils etc. Let me think... Lentils are around £1.50 per kg.. That's enough for over 10 meals.. 500g of pasta is about 30p... I think a kilo of supermarket budget rice is about 70p. (I'm basing this off the top of my head at Morrison's prices..)
So say 500g of lentils at 75p, 500g of rice at 35p and 500g of pasta at 30p. Now you really don't need more than 100g of each to make a meal. So that's £1.40 on those.
Frozen vegetables are your best friends. I can't remember how much the value ones in Morrisons are but I've just checked on sainsbury's and they're 72p per kg. Use them in a soup along with the lentils, stir-fry them up and have with some rice (my staple meal.. I figured out that including the oil and soy sauce, it costs me about 9p per portion..)
Tin of tomatoes (400g) is about 30p. I would use that in two meals but you could make it last 3. (Mix them up with the pasta)
I don't know if you can use store-cupboard things that you already have. If not, get some really cheap veggie stock cubes (sainsbury's have 10 for 10p), some cheap mixed herbs...
Umm.. for lunches, instead of sandwiches how about couscous? Again from sainsbury's.. 500g is 61p. That's more than enough for 5 lunches. Fry up some veg (I usually use garlic, red onion and red pepper but the cheap frozen veg will work just as well), whilst you're frying it, make 80 to 100g of the couscous up with a veggie stock cube in the boiling water. Once the veg is cooked, separate the couscous with a fork, mix in the frozen veg and add some mixed herbs.
Or I suppose you could get a basic large loaf (800g) for about 60p (I think), then you could have toast for breakfast as well but I don't know what you would do for fillings for sandwiches...
As for what to drink.. That's the easiest thing to do cheaply. Just drink water. Or if you get bored of water get some of the supermarket value squash.. Say 30p for enough for a week?
OK... with 500g of couscous, rice, pasta, lentils, 1kg of frozen veg, 10p for stock cubes, 1 tin of tomatoes and 1 bottle of squash I make it £3.43 on the prices I've put..
So that leaves £1.57 for things like herbs/spices, salt, pepper (again I don't know if you're allowed to use stuff you already have in your stock cupboard or not...)
I would say, you are vegetarian which is great.. However, dairy, eggs and mock meats are stupidly expensive. Seriously, just don't eat dairy or eggs for the five days. Five days eating a near vegan diet won't kill you.
I mean I can live off about £6 a week (I usually spend £5 a week and then every month spend an extra fiver (or a tenner if I don't buy the really cheap stuff) on stocking up on herbs, oils etc etc ) for food. I do eat a lot better than what I've put above but like I said, I don't know if you're allowed to use stuff you already have or have to spend £5 on everything you eat in those 5 days including things like salt and pepper etc which is why I left some money over....