Drop a brand. Weetabix is weetabix regardless of the box.
Stop buying expensive out of season fruit and veg. Bananas, satsumas/clementines and apples are all cheap. Frozen berries are cheap to add to porridge.
Meal plan based on deals ie 3 for £10 meat buys us sausages, a chicken, some pork, so 3/4 dinners' worth of neat for £10!
Add lentils or veg to bulk out stews and spag bol.
Reduce meat.
Don't buy dessert - bananas in custard/icecream are cheap. Yoghurts are cheaper.
Only buy what you actually need, regardless of how good a deal it is.
We are a family of 4, 2 adults, 1 hungry teen and 1 hungry toddler and we spend about £50 a week and eat well.