rubik, there are plenty of edible plants which grow wild in Britain, as well as commercially grown crops. It has to be said, though, that while they'll keep you alive, not all of them are particularly toothsome. Dandelions, daisy leaves, ransomes, bittercress, shepherd's purse, nettles and many more can be made into salads or cooked, yet few bother because they taste bl**dy awful!
I suspect most would become very tired of foods produced excusively from British produce in short order; my relatives who survived the two world wars certainly did.