Before shopping
Check cupboards, fridge and freezer, plan a few meals and write a short list. Build at least one meal around food that needs using first.
In the shop
Compare unit prices where useful, consider own-brand alternatives and only buy larger packs when you are likely to use them. Reduced items are only a saving when they fit your plan and can be stored safely.
During the week
Use leftovers safely, freeze suitable portions and test one packed lunch or lower-cost meal rather than trying to change everything at once.