Finding a Sangha in London
I've been to London Buddhist Centre and that was very good. It's part of the Triratna Buddhist Community and it's explicit aim is to present the Buddha's teaching in a way that is compatible with modern society. It's in a converted fire station so it's a really good space and it was friendly when I went.

I am biased though - I'm involved with Triratna in Leeds so it's my sangha. I think it's still a good recommendation though.