Unfortunately, I don't think that you'll find any food stores accepting PayPal.
PayPal take a much higher percentage commission than the credit card firms do. If the food stores accepted PayPal, all of their profit would be taken up in fees. It's only luxury goods retailers who can accept PayPal. They have much larger profit margins and can afford to pay the fees charged by PayPal. This seems to be confirmed by taking a look at the UK companies which accept PayPal:
http://www.paypalgrandgiveaway.com/allshops.as px
It looks as if you'll have to find someone to lend you a few pounds to see you through the next few days. With some careful budgeting, it's easy enough to live on a pound or so per person per day, so you shouldn't need to borrow very much.
Chris