Slow cooker. You can buy one for £25 & then they cost pennies to run.
Use it to batch cook lots of cheap vegetables into sauces, soup etc.
Bulk mince out with lentils to make Bolognese, chilli etc.
Buy chicken thighs - really cheap & you can make amazing curry (again in the slow cooker).
Buy big, economy brand pasta & rice. Potatoes are also good value - buy them loose.
It won't necessarily be exciting, but you'll be able to eat every day!
If you have a freezer, freeze any leftovers however tiny the portion. Then you can make a pasta bake or a biryani from several pots of leftover pasta sauce or curry.
Homemade cake for a treat rather than buying chocolate etc - you can put just about any leftover fruit in it - just use a basic carrot cake recipe but throw in things like apples, bananas or grapes - in my house this is where the uneaten fruit from lunchboxes always ends up!
Natural yogurt with a spoonful of cheap jam rather than buying posh fruity yogurts.
Buy cheap shampoo but decent conditioner. Bar soap not shower gel.
Only heat one room in winter - & get everyone to wear warm socks & a jumper. Cold bedrooms are fine as you warm up quickly once in bed!
If you drink, go own brand on vodka & gin. They are fine whereas cheap wine & whisky are just nasty!