Sticking with the London theme ..... Southwark Cathedral.
Thought to be founded as a nunnery around 606ad by a maiden named Mary on the profits of a ferry across the Thames she had inherited from her parents. Later, about 818ad, converted into a college of priests, refounded as an Augustinian priory, and then serving as a parish church before consecrated as a Cathedral in 1905. The early dates are thought to be optimistic.

The space is incredible, the location, next to Borough Market, is fantastic and the recently installed artwork, Doubt, is striking (see extra).