Back to basics for you.
Breakfast is cereal, toast or porridge.
Lunches are sandwiches (ham & cheese in particular you can buy in bulk) and make a big pot of vegetable soup which costs hardly anything to make - barley, soup veg, veg stock, water - bam!
Dinners are simply things which are cheap to make - anything with something carb heavy, e.g. pasta dishes, casseroles, pies, etc. If you have leftovers, keep them frozen or on fridge for another day.
Snacks are cheaper bits of fruit (apples, bananas, satsumas) and some veg like carrot sticks, celery etc. For something "bad", the own brand of any supermarkets usually have biscuits/crisps etc reasonably cheap and you'll just need to ration it out.
Buy the cheapest brand of any product you see.
Make sure you cut down a little on portion sizes and ration snacks to last a full week/month.
In terms of fuel, you calculate how much you need for essential travel. Anything outside of that, walk to the closest shop.