Do a joint for dinner at least once a week - leftovers are great for sambos and other meals. Bacon or chicken are especially good, but beef and lamb make good leftover curries and nice sambos. And leftover curry could be used in a pasty type thing for another lunch. A little leftover meat with some cheese and maybe something like mushrooms or peas in an omlette is another tasty lunch-sized meal, or even dinner if with salad or baked potato.
While I don't buy everything in the Value/Basics ranges, things like pasta and rice are fine from those, and I find Sainsbugs basics tinned tomatoes are great, for example.
Lidl has some great things too - chorizo sausage is very useful and lasts a long time (I make a potato bake using half a chorizo, 1 portion of Lidl bacon pieces, pack of salad potatoes and jar of tomato and chilli sauce - which is very tasty!!). Bacon pieces are great too to add enough meat and flavour to a mainly veg dinner (mac and cheese with loads of veg, omlettes, over baked potatoes, add to stuffing for tomatoes,,). Their plain pizza is very handy, and you can throw any bits and pieces you have onto it.
Making your own bread and cakes, pizza bases, scones, biscuits etc is a great money saver.
Tomato sauce for pasta is very easy to make, and quite a bit cheaper. And can be frozen too if you make it in bulk.
If you have an asian supermarket near you, many things will be cheaper there. Rice, spices (you pay the same price as supermarket but get much larger packs), vinegar (for chips, but also as a cleaner and for adding to laundry instead of fabric conditioner), tins of coconut milk, large tins of tomatoes and tomato puree, puppodums and loads of other things.
If you don't finish a loaf of bread, whizz it up in food processor to make breadcrumbs and freeze. (If you have time, make into a plain stuffing mix before freezing as it's handy to have a bag of that ready, as well as plain crumbs). Great for coating things (like making your own chicken nuggets or fish fingers - or crab cakes using a tin of crab, tin of corn and leftover mashed potato), but also for stuffing a joint, putting a topping on pies (like chicken and mushroom - could be fresh chick or leftovers from joint), or making something like stuffed tomatoes and mushrooms in a hurry for dinner one night. You can easily change a "standard" (onion and herb) stuffing you have frozen to suit the needs - add bacon and cheese for tomatoes, fried mushroom stalks and a dash white wine for mushrooms, chopped apricots and apple juice for pork, rosemary and garlic for lamb, etc.