This is quite the claim. Do you have any evidence that this is at all common? On the surface it sounds like a conspiracy theory tinged with racism.
I’ve never seen a claim like this levelled at London, famously a place increasingly losing social housing stock for luxury housing. The only thing I can think of is this is a new twist of the complaint that a lot of London’s housing is left unoccupied because it’s Russian/Saudi vacation homes or whatever.