Earlier quoted context omitted.
Note that £20-45k/month (US$25-60k/month) is more like the going rate for these properties. http://www.nestoria.co.uk/mayfair/property/rent/2?price_min=... But either the owner isn't interested in the money/hassle/attention, or occasionally uses the property — it's difficult to show off your fancy London apartment if people are living in it, and you can't casually lend it to friends or relatives.
I reckon people with $25-60/month to rent out these properties would already have enough money to buy their own homes slightly away from the city center
I pay $2k to live in a city centre. I could pay half that to live at the edge of the city, but then I'd be at the edge of the city.