Brexit will be a disaster for the UK, but it won't slow population growth or immigration, which means it won't slow London's growth. The UK is going to remain the fast growing country in Europe. It will just result in a poorer country with a larger proportion of non-white residents, as a larger portion of immigration comes from outside of the EU.
This is a post-Brexit vote article and projection:
https://www.theguardian.com/uk-news/...l-figures-show
http://www.ippr.org/files/publicatio...f?noredirect=1
The key is going to get Brits more comfortable with high rise or apartment block living. I am entirely against building on Greenbelt land, and also don't want to see rural areas ruined by sprawl. There are lots of abandoned rail yards and industrial land, or unremarkable postwar housing that can be densified around London.