Borough Market, minutes from London Bridge, is London's oldest food market and it is amazing - it's the most delectable smorgasbord of tastes, smells and sights and will have you breaking every diet rule in the book as you wander amongst all the stalls taste-testing all the wares. I actually didn't need to buy any lunch for myself as I was so full from sampling cheese, chutneys, tapenades, sausages, falafel, cured meats, muesli, chocolate brownies etc etc (it really is the perfect destination for skint working travellers in this respect!) Just watch out you don't spend as much money as we did on purchasing a gourmet selection of the things we sampled (hhmm, I'm thinking this is the whole point of taste-testing, no? Crafty buggers).
You can get every imaginable type of cheese (yes, I had a field day), all kinds of fresh produce from France, Switzerland, Germany and more, plus fine wines and pate's, and weird stuff like haggis, fresh hare and pigs feet - mmmmm. There's also some great deals to be had at the fresh fruit and vegetable stalls, if you're over buying sick-looking veggies from Tesco and Sainsbury's. We got about 60 of the reddest, ripest, most delicious vine tomatoes for a pound - bargain! Will definitely be going back.