Two doors down from my workplace (Natural History Museum) is the Brompton Oratory, a big Catholic church for the South Kensingtonians - Catholicism is relatively rare for Protestant England.
During WWII it functioned as a 'dead letterbox', where spies would leave their missives behind a loose brick.
To the right of the building is a nondescript alley - this is a gate to a hidden world. Follow it and turn left at the end, through the church grounds and you'll come across a village green.
Directly behind the Oratory is the Holy Trinity Brompton, the Protestant version
Surrounding the church green are a maze of mews streets, in short a secret village hidden from the main roads and crowds (this part of Albertopolis, a massive complex of museums and universities). The mews are former stables for the mansions behind, now converted into tiny $6 million homes.
The secret park where we drink at the uni bar (also site of the Iranian Embassy siege in 1987). It's a delight to lie on the grass sipping beer with your mates.