It's like a half of Russian mafia owns mansions there, right in the open, laughing in the face of UK courts, and its money laundering laws.

>an expensive area

This is a bit of an understatement. If Mayfair is not the most expensive neighborhood in the world, it's close. A large number of the residents of Mayfair are former soviet state security who became oligarchs with the money to live there by pillaging their country then funneling that cash to the west with the aid of western banks including British banks.