I really heavily on veg and pulses.
Red lentils are £1 a pack and 1 pack easily makes a lentil soup ( add a tin of tomatoes (31p) and a handful of spinach (£1 a bag)
And a dhal (add onion (3 for a £1) and spices / curry powder) make basic flatbreads (plain flour 65p and water)
So that's two meals for four with leftover flour, spinach and onions.
With the spinach and onions - add to tortilla/frittata (eggs £1)
Three meals for 4 under a £5.
The same goes for chickpeas- dried bags are £1 and make 2/3 meals. You just have to soak them overnight- then you can make a chickpea curry, chickpea "meat" loaf and a chickpea and lentil stew (onions, tomatoes and dried herbs/ spices needed).
Butter beans are £1 for a pack dried - make mash, burgers and a tomato bean stew. (Again onions, tinned tomatoes and seasonings needed)
I'm always confused at people who say it's expensive to eat vegetarian - veg and pulses are shockingly cheap compared to meat! First thing I do to save money on shopping is cut down on meat, and increase the pulses