Leadenhall Market is a covered market in London, located on Gracechurch Street in the City of London. The market location dates from the 14th century and the present listed buildings are grand. Allegedly this is a produce market with butchers and veggies but we could only see expensive fashion, bad chain food and vast amounts of people in suits drinking in Ye Olde pubs. Best not to go at 1pm on a workday I suspect.
Wikipedia says that the cute old passages here were used for Diagon alley in the Harry Potter films.