[throwaway2a02]

In most places, including the UK, there's a legal principle called adverse possession (https://en.wikipedia.org/wiki/Adverse_possession), where anyone can take possession of a property for free, if it's unoccupied/unmaintained by the owner, and the possession is not challenge for certain a period of time, i.e. actual owner doesn't evict the prospective occupier (12 years in the case of the UK).

So people or the government don't really need to actually buy much of the land.

[arethuza]

That link actually only talks about England and Wales. Scotland has positive prescription:

https://en.wikipedia.org/wiki/Prescription_(Scots_law)#Posit...

Mind you the end result is similar - different legal systems though!