There is a great deal of overseas wealth in London which is forcing up property prices.
Flats (apartments) and houses are bought and perhaps occupied for a few months a year, but are very often left empty waiting for the price to go up.
Much of the foreign 'investment' comes from Arabs (oil), Malaysians (property), Chinese (industry) and Russians (don't ask).
I suppose that if you can only get 0.5% on your money from the bank, you might as well 'park it' in property and watch the value increase by 5% pa.
There's a good article from the Guardian here:
Londoners miss out as homes built as ‘safe deposit boxes’ for foreign buyers
And another one from the Daily Telegraph here:
These are the foreigners ruining our country
On the face of it, it's a rather inflammatory headline, but do read on.