There certainly exists properties (even, anecdotally, entire streets) in London that serve as wealth-deposit-assets for rich foreigners of questionable repute.

There certainly exists properties (even, anecdotally, entire streets) in London that serve as wealth-deposit-assets for rich foreigners of questionable repute. Many of these are left empty as the owners aren't in the "landlord business" and merely want to hold a liquid asset (an occupied house isn't as liquid as an unoccupied one). There is also a story about the saudi royal family buying up a strip of houses on Bishop's Avenue (the most expensive street in London, despite being somewhat suburban) sometime in the 80s or 90s as a bolt hole, but IIRC recently disposed of. It's clear that this produces some pressure on the housing supply in London, but nowhere near enough to account for the sheer numerical pressure (some ~800k housing units required by 2021 and the rate of new builds is way, way off that target).