Onions + mushrooms + tinned tomatoes + herbs (some combination of sage, thyme, parsley, marjoram and oregano is best. Two are usually enough) + chilli and/or tonnes of pepper + garlic + whatever veg/meat/fish you have mixed in (optional). Add to pasta. Cream over yourself. All the ingredients are cheap, and you can make an absolute tonne of it and eat it over the next several days to a week (depending on meat/fish content).
Omelettes with chisps and salad/peas.
Baked tatties with beans - lots of protein, carbs, and some sugar for energy. Add butter/margarine and you've a bit of fat and vitamins.
Sausages (don't go bottom of the barrel, but one up is fine) sliced and fried in cajun + pitta bread and peppers/salad/rice.
Fried onions with sage added to grilled sausages in a part baked baguette.
Frozen beef burgers grilled and topped with tinned pineapple, with peas/beans/rice
flour + milk + a little sugar. Make a batter (consistency of thin porridge - slightly melted mcdonalds milkshake) and dip tinned pineapple in it. Fry on both sides for a cheap, tasty and relatively nutritious dessert / snack.
Frozen beef mince can be deceptively tasty (and the consistency actually makes for easier sauce-y things like bolognese than fresh mince), non-shitey, and good value. It's been ages since I bought any though, so I'm not sure which to recommend.
Mackerel (sp?) or kippers can be grilled for your fish fix - they're cheap, and not the most delicious or versatile fish, but they're good for you. Possibly an acquired taste, as they're powerful. Anyway, they're a good source of some fairly difficult nutrition when you're on a budget.
Check with local butchers if there are any. You never know what they can turn up, and most are honest enough - they need to be. They're a better source for bargains and discounts than the supermarket, also.
Porridge! Oats are cheap as hell, and with a bit of sugar or honey (oh, honey is a wonder food, by the way. If you can get honey and/or yoghurt at a good price, they'll be the best bargain of your life - they're ridiculously good for you as long as you don't go nuts with them. And an unfinished jar of honey will outlive you if you seal it), porridge becomes a much less depressing breakfast. It'll keep hunger pangs away for the best part of a working day, and leftover oats can be used for baking if you're so inclined.
Baking! The easiest thing in the world to bake is a flapjack. Melt butter in a saucepan and add golden syrup (roughly equal proportions - salted butter is fine). Add oats and stir together until the mixture is very, very thick with oats, like moderately congealed porridge or thick cement. grease a baking dish or cake tin (or narrow-ish casserole dish) and pour the mixture in. Bake for about half an hour and allow to cool (flapjacks are one of few baked things to be better when cooled), and you have a cheap and delicious snack/dessert that'll be good for a week and edible for longer. It's important to treat yourself, so don't feel bad.
Speaking of part baked baguettes, they're a bloody godsend sometimes. You can get two for about a quid in any supermarket and even most corner shops - shove them in the oven at about 180 for 10-15 minutes, or higher for less, or lower for more, it really doesn't matter as long as you check on them now and then. They can be filled with anything and make a great lunch, or split in two or three for rationed snacking, or broken and used as dipping devices, or filled with chicke or tuna or egg or sausage for a filling, cheap and piss-easy meal.
Sometimes you're not actually hungry at all, but just want the comfort and distraction of having something to hold, chew and eat. Apples are great for this, as they're satisfying to hold and manipulate, satisfying to bite into and chew, and are quite solid, and though not the most nutritious of fruit, still better for you than biscuits or a pastie or whatever.
Er... anyway. Shutting up, now.