Leadenhall Market

Leadenhall Market is one of the oldest marketplaces in London, dating as far back as the 14th century when it was established as a place to sell fish, meat and herbs. It is also situated in what was once the centre of Roman London.

 

More here

(What is not mentioned in the article is that the market was used as Diagon Alley in Harry Potter and the Philosopher’s Stone. The entrance to wizard’s pub, the Leaky Cauldron, is in real life an optician in Bull’s Head Passage in Leadenhall Market.)