Value kidney beans, chopped tomatoes and sweetcorn are all identical to conventional own brand versions.
With meat, rather than trading down to value (please don't, think of the animal welfare) trade down on cuts. I haven't bought chicken breasts for months now, because 4 free range chicken breasts now costs about 8 quid and I just won't pay it. Buy drumsticks/thighs instead and make a yummy casserole. Buy neck of lamb and chop it up instead of diced lamb.
I have halfed the quantity of meat I put in ewverything but bulked it out and so far DH has not noticed.
Sliced cooked roast meat can be frozen. To defrost, take out frozen, put in an ovenproof dish and pour over some gravy, whack in oven for 10 minutes or so.
I now chuck a tin of chickpeas into practically every dish I make.
When making bolognese, you can make 1lb of mince stretch twice as far if you put in extra tin of tomatoes, an extra chopped pepper and a few more mushrooms. Freeze half. I do the same with shepherd's pie/cottatge pie.