Hiya
I also have a budget to watch! I buy basic ingredients to add to for meals - tesco do huge (2kg?) bags of pasta for £2 I think, and a supermarket's own 5kg bag of potatoes will do for ages - they generally make decent chips, mash and jackets. Always compare the price per kilo/100g of product, sometimes it can be suprising what is cheapest. Likewise Huuuuge bags of rice can be cheap, although in my experience at lidl the smaller packets are actually cheapest on weight for weight.
You can get away with a lot of ingredients at Tesco value/lidl/aldi/asda smartprice/sainsburys basics brand level without noticing a difference in quality. In my experience this includes tinned tomatoes, passata (brilliant basis for soup and pasta sauce and free), fake smash, tinned peas/sweetcorn/red kidney beans/spaghetti hoops/baked beans/potatoes/dried spaghetti & pasta, mixed herbs...
Try swapping similar things depending on your findings - for example any recipe that wants chickpeas/butter beans etc could work with kidney beans (I tend to find they are the cheapest canned pulses, at about 15-20p a tin), cheapest brand dried spaghetti is generally cheaper than any other pasta, so can use that instead of penne.
I tend not to eat meat at all if I'm skint, but you can bulk out meat to make it go further. Apparently grating carrots into anything made with mince is a tried and tested way, and most people say their families are none the wiser. Stews and soups are your friend too - a little meat goes a lot further with lots of veg added. Buying a whole chicken is generally the most economical way - roast it, use the meat and boil the bones to make soup afterwards.