Currently I live on about £30-40 per fortnight - I shop every two weeks as I dont really eat much fresh stuff - when I do I eat it all in the first few days after the shop.
This doesnt include the odd few sandwiches and £3 meals at cafes at uni etc, call that £20 per fortnight (maximum) - brings me to about £60 per fortnight or £30 a week ... not bad - most of it is fairly healthy too (no super/pot noodles, crisps, chocolate, fizzy drinks etc).
On a side note, I once did a challenge at uni where me and a couple of mates lived off £1 a week each, basic rules were set, no handouts from people not involved, must begin with absolutely nothing, we clubbed together and bought a reasonably sized bag of asda pasta, loads of 8p super noodles and cans of 6p beans amoungst other bits - we actually managed it! Wouldnt say it was easy though, the temptation to just splash out a couple of quid on a tasty sandwich was immense by the weds/thurs (friday wasnt so bad as you only had 2 days to go)!