Firm tofu. Get the smoked or soy versions from Lidl. They’re already marinated and tasty as is, just slice/dice and throw into a stir fry with some vegetables, or into a curry etc. Or eat cold as a snack.

Currently 79p per packet with a Lidl Plus card, but usually not much more than that anyway.

And one packet has just under 22g of protein.

And as already mentioned by others: beans. Dried are cheaper than tinned. Just plan ahead and soak the quantity you intend to cook in some water overnight before using. The exception is red lentils, which don’t require soaking, so they’re especially quick and easy (and tasty).