lots of tinned tomatoes, pasta, mince, jacket potatoes eggs and beans are the key staples.
A bag of about 6 or 7 baking pots is like £1.60, they are not huge, stick half a tin of beans on top and some pepper and presto theres the potential for 6 meals there, you are not going to get fat but it will feed you. as said porridge is around £3 for 2Kg from morrisons, this will feed breakfast for the best part of a month though with no budget for milk i suspect youl need to whack in a little sugar.
of course there is beans on toast, if you have a cheap farm house shop near by this would help dramaticly as you can get like 30 eggs for £3 in some places, that gives you the potential for lots of eggy bread for a twist on beans on toast, omelet's or scrambled egg on toast with some tommy sauce is great, variations are scrambled egg and beans etc.
as others have said, spag bol is very effective in bulk. go student and you can have spag bol toasties if you have a toastie maker, bulk it up with a tin of beans, whack some in a dish and cover with mash and you have italian cottage pie, very tasty
it is very doable, you will be great for the first 2 maybe 3 weeks but you will see some weight loss and start to get sick of your diet and the manotiny. The key is you will have to be creative.