"There are also quite a lot of Cornish residents unable to buy at all, never mind sell again."
Boo hoo, that's the same for any popular/desirable city or area in the world.
E.g plenty of people who grow up in London can't afford to buy there but plenty of times on MN are just told to move to a cheaper area city.
Plenty of people who grew up in the south are advised to move north for a cheaper home when complaining about southern house prices which are driven up by demand of people wanting to live there.
You know what plenty of people in the UK can't afford to buy because of the increase on housing demands which is partly due to an influx in immigrants (both rich and poor) who come and buy property. Yet any notion that they shouldn't be allowed to it met with cries of racism.
So why are 'Cornish people' a protected species who should be allowed the unrivalled right to buy in the areas they grew up in.
I, for clarification, don't think we should stop anyone buying a property because they're not 'from there' as this restricts free movement and is a move towards both state control and othering of any outsiders making people hostile.