Xenia, your argument seems to hinge on the housing market in this country being "free". It is anything but free.
The available land for house building in this country is incredibly scarce. The price of land has increased tenfold in the last 10 years, mainly because land with residential planning permission is like gold dust.
If it was a free market, there would be no problem of housing, because enough people would build homes where they wanted to to meet demand, and the price would fall.
Where there is a bottleneck of supply like this, there are many options for how to distribute the scarce resource among people needing to use it.
It is more analagous to the supply of water in the South East of England in times of drought, or congestion on busy roads. As far as I know you can't buy your way out of a hosepipe ban, or out of a traffic jam.
A first home to live in is not a luxury, it is a fundamental human need. A heavily constricted supply situation and rapidly increasing demand has created an opportunity for speculators to make money from anyone who needs a home after they have driven prices up.
Any multiple occupation of the housing stock (second homes being a perfect example) is reducing the overall stock available for use by someone else - regardless of how large or small it is.