Where Is This Again?

The setting is modern London.
Personally, I think the setting is great because London is a relatively large and mysterious city. I know of a lot of stories that take place in London, and they are fantastic.
However, this story takes place in two Londons. London Above is our normal world. This is where Richard originally was, and where he found Door.
And then there's London Below, and alternate reality where, if someone from LA enters, they are erased entirely from it. Anyone from LB is hardly seen in LA. It takes a lot of focus or the LBer conversing with the LAer.
The way you enter LB is through a few special manholes.