Agree with most (maybe all) of Frances' suggestions. In particular, Borough Market (at the south end of London Bridge) is a must destination for me. But the full, farmers market is only Fri and Sat. The rest of the time there's a few permanent stores, so maybe not worth a detour. http://www.boroughmarket.org.uk/

But the food selection is amazing - but be careful if travelling with 'persons of a nervous disposition' - when I was there a couple of months back there were numerous 'hanging game' for sale. In addition to the birds, there were 3 different types of deer - the complete animal (missing the head). Easily the best place in London for lunch - but very long line-ups too. It's very popular.

Alan Gardner