dried beans are a really good source of cheap protein and a 500gm bag will feed at least 6 people depending on what you do with them.
Green lentils are good for substituting for mince, they hold their shape and texture a lot better than red ones and don't go mushy.
Split peas make good soup and dahl and are totally cheap.
Potatoes can make a complete meal if you make patatas bravas(espec. if you add some sliced chorizo.) cheese and potato pie or potato bake and you can use left over mash to make potato cakes or bubble and squeak.
Spanish ommelette; you can make this for dinner and eat left over slices cold for lunch.
Buy a chicken, cook it and divide the meat into 3 lots to use for 3 meals like risotto, chicken and veg pie, curry, chicken pasta bake. Make stock from the carcass and make soup with onions, carrots, leeks and barley or rice.
Root veg casseroles with a handful of barley added for protein or 100gms of dried kidney beans, soaked and cooked for protein.
Rice based meals like cheese and tomato rice, rice and beans, lentil and rice tacos, mixed veg savoury rice are all quick easy and cheap.
Pasta with things like a tin of condensed mushroom soup, some corn and tinned mushrooms is popular with my kids, as is pasta with a sauce made with pasatta, garlic, onion, paprika and chorizo or pepperoni.
Chowder made with onions, leek, potatoes, 1/2lb of smoked mackerel and a couple of pints of milk (made from dried milk)is also popular here. Or replace the mackerel with a couple of frozen white fish fillets cut into chunks and add some celery and carrots to the veg (this is my youngest's favourite soup.)Serve it with crusty bread or hm soda bread and it is a very filling, hearty meal.
Another very popular thing in here is this:
3 cups porridge oats mixed with a couple of eggs, water, poultry seasoning and sage. Mix it all up and shape it into 6 large burgers. Fry until golden on both sides. Mix a tin of mushroom soup with a tin of milk, pour over the burgers and heat through gently until the sauce is a bit thickened. MY kids love this with mash, green beans and carrots. I know that it sounds weird, but it is nice.
Another popular thing in here is a pork shoulder meal. I buy a big one and slow cook it. The kids eat half of it and then the other half goes back in the slow cooker or a casseroel dish in a low oven with about 1/3 bottle of barbecue ketchup and a finely chopped onion. I cook this for a long time and then shred the pork between two forks until it is completely pulled to pieces and the sauce is mixed right through it. The kids love this in rolls and because the meat is shredded it goes really far.
Minute minestrone is another popular thing in here. Boil 2 pints of stock, a chopped onion and a tin of tomatoes for 5 minutes or so. Add about 1/2 a pack of tiny pasta and cook for a few minutes. Chuck in 1/2 a pack of frozen mixed veg and simmer for another few minutes. Stir in a few spoonfuls of pesto and serve scattered with grated cheese. Just adjust quantities to get the consistency you like.