When things are tight we look in the freezer and get inventive.
We shop at Ocado and Sainsbugs because we don't have a car, the bus is a faff and they both charge the same in delivery as a cab fare anyway 
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We rely on frozen veg a lot because if we buy fresh it's always packed in large quantities which we struggle to eat before it goes mouldy. There is only three of us you see. When we buy frozen, we can choose exactly how much we need from the bag and put the rest back for another day. No waste. I read that the nutritional content is the same or better.
We buy chicken fillets from Ocado in bulk, around 10 for £12, which we freeze and take out and defrost as needed. We buy other meat in those 3 packs for £10 deals, and because we are a small family, sometimes we can stretch it two days. Chickens are brilliant. We buy a largish chicken to roast sunday, leftovers are put in stew or soup Monday, then dh boils the bones with some celery and carrots for stock during the week.
As with another poster, potatoes are getting too expensive so we eat more pasta and rice these days. It lasts for months and the more you buy the cheaper it is.
Ocado do frozen garlic, herbs and shallots and we use them for seasoning, again, limiting waste. Our cupboard is also full of various curry pastes, passata, tomatoes, differing species of beans and lentils, flour, sugar, pasta, rice and other ingredients with long life.
If we are in very dire straits, we go veggie, use as bit of leftover curry paste, tin of toms and some tinned chick peas plus some frozen mix veg and make curry. We also curry up baked beans with some onion and mixed curry spices, served on a naan or chapati. Beans and Chick peas hold some protein, though not as much as meat, but still.
We eat as lot of porridge. Cheap and filling.
We buy all dry, tinned and long life food monthly, then fresh food during the week. The monthly shop is around £100 and the weekly around £20-30. So top whack is around £220 a month on food. We live in London and we don't do too bad.