Pasta bake. A jar of pasta bake sauce from about 80p in supermarket, a couple of handfuls of any frozen veg, fresh veg or a small tin of sweet corn or peas, sometimes I add bacon, tuna, beans or baked beans, a couple of handfuls of grated cheddar or mozzarella for the top (finely grate to make it go further).
Mash potato topped pies. Mince for shepherds or cottage pie if you have enough space in the budget, cowboy pie is great too.
Bangers and mash with onion gravy, add some Yorkshire's and some carrots/other veg it serves as a decent substitute for a Sunday roast.
A roast chicken. Eating it as is doesn't make it go very far, but strip it down and use in stews, curries and casseroles it's the cheapest way to buy chicken. Drumsticks and wings make great snacks.
Basing meals on lentils and beans. Bean chilli, lentil and vegetable soup, dahl, etc.
My cheap basics are bread, peanut butter, eggs, baked beans, beans, tinned tuna, frozen veg, flour, milk, oats.
Porridge is really filling but you can also make baked oats, overnight oats, crumble, oat pancakes, and use them to thicken most things. Split peas and lentils work well to bulk meals out too, or barley.
Peanut butter is high fat and protein and works well to add to porridge or toast to make them much more filling. Can also be added to curry or stir fry and to make satay sauce as well as baked goods.
Eat bread with everything. It's not great long term, but short term having a couple of slices of bread and butter with cooked food fills you up a lot. Also other cheap carbs like potato, pasta, noodles, cereal.
Fruit is expensive and so are snacks.