On Friday afternoon and Saturday, the Borough Market is open the the public. It is located under the railway, and is one of London's oldest and largest. The design is Victorian.
We were staying in an apartment with a fully-equipped kitchen, so we made good use of the market, buying organic lamb chops, the thinnest asparagus I'd ever seen (smaller than a pencil), and tiny yellow potatoes to make for dinner. But even if you don't have the use of a kitchen, you can still enjoy the fresh baked goods, fruits and vegetables, and prepared meats, such as pate and dry sausages. Besides, it is always fun to look at unfamiliar produce, fish and meat.
Just outside the market's roof is Neal's Yard Dairy. You will probably smell it before you see it. It's the place to get a sample of some wonderful English cheeses, hard and soft. (And they'll gfive you a little bit to taste, if you ask.) We bought a piece of something called Stinky Bishop, and it really lived up to its name. They also have delicious bread from Poilane, one of Paris' best bakeries.
The stalls vary and move around within the market from day to day. We went on a Saturday the first time, and were disappointed that some of the sellers were not there the following Friday (an exotic sausage maker, in particular). So if you see something you want, buy it, because it mught not be there the next time.
Tube stop: London Bridge, on the Northern line (toward Kennington). Hours: Friday noon to 6 p.m., Sat. 9 a.m. to 4 p.m.