Spag bol - a pound of cheap mince, 4 tins of tomatoes and a pepper (plus the spaghetti) will give you 3 to 4 meals for a total cost of under £3 (depending on the kind of mince you go for). It freezes well too.
I'm also a fan of lentil stew - half a cup of lentils, 400g of diced beef, 1 tin of tomatoes plus a few herbs (bay leaf, mustard etc) and an onion and a carrot - 3 meals for about £4.
Chili - similar to spag bol.
And tbh, pasta has so many potential variations - one £1 pack can last you a week with a variety of fresh veg, frozen prawns, leftover sausages etc.
Oh, and last week I got some cheap chicken thighs, put some honey and orange on top, baked them in the oven til they were really sticky and had them with mash. All for under £2, very tasty and I felt like a cooking god.
Don't get sucked in by readymeals, as it's usually cheaper, and not much more difficult, to cook it yourself from scratch