There used be several in London but I assume gentrification has changed that.

Not really. I think it boils down that most cities (and their areas) in the UK are mixed: so you have few streets of expensive houses, and next to it could be a council estate.. In Paris for example, it's not so mixed and you will get whole districts that are poor and deprived. These districts are usually inhabited by immigrants, and forgotten by local govs. Hence the so called "no-go" zones.