Assuming they don't cause you any trouble, oats are a good and very cheap carb source.
When my budget gets tight, I tend to add oats+whey wherever suitable to get more calories in.
Ground meat is a good and relatively cheap source of protein. Waitrose usually has 2*500g of ground pork for £4. Depending on your needs leave the fat in (8%) or de-fat it. Ground beef is a bit more expensive (depending on the quality), but I like heaving at least some every day.
As much as I love eggs they are relatively expensive in this country, so they wouldn't be my first choice for a protein source when money is an issue (unless you can get decent once for a reasonable price somewhere, but I think even something like Sainsbury's Basics eggs isn't good value for money and they taste awful).
I think milk is pretty much a waste of money. Lots of crap in it and doesn't really provide much protein or anything (you can get your saturated fat from meat).
Here are my staples:
ground meat (beef, pork)
oats (my main carb source)
frozen vegetables (chopped spinach, broccoli, cauliflower)
almond butter (peanut butter will do fine on a budget and is cheaper, but the difference would be maybe £5 a month, £10 if you eat a lot of it)
coconut oil (one tub lasts me a month and costs £8)
sweet potato (not a cheap carb source, but I have one 500g a day for about £1. Provides about 100g of carbs and a bunch of other good stuff)
water (tap, at uni Brita filtered)