I'd say about £100 pw for two adults and 2 children (5 and 3).
I buy Tesco Everyday value on lots of things...lots of tins like tomatoees, tuna, sardines. The value veg and fruit (most of it...not strawberries, NEVER strawberries from Tesco even in high season, they are shit)
Some things I will only buy branded...butter, yoghurts, bread, cereal (except Weetabix...I much prefer cheap Weetabix as it's firmer) being a few. I will only buy free range eggs and chicken.
I do buy the 3 for £10 on pork loins...£10 worth of those is enough for 4 meals for us all (either just oven baked meat or chopped up in a casserole or stirfry)...they are brilliant value and as nice as any expensive pork loins I've tried.
I do buy a lot of berries which are expensive (not strawberries obv lol). I also buy at least one nice cut of meat a week, usually for Sunday lunch. Also quite a bit on herbs (dried and fresh). I also spend about £10 a week on cheese which puts the bill up...I have a dh and two kids who are complete cheese fiends and love all the expensive stinky cheeses, and in the case of the dc, brie, which isn't cheap.
If I had to, I think I could still feed us decently on half that...we'd reduce the meat we buy and not buy free range chicken/eggs, switch to budget brands of more items, stop the range of cheeses that fill our fridge.