All that happened is that councils stopped building homes and sold some of the ones they had to current occupants (and councils actually also have control over planning permission, so they did this and blocked anyone from building new houses at all).

There is other stuff going on (it is likely impossible for the UK to build enough housing given demographic trends atm) but this is all caused by not building enough houses (the reason why investors are buying houses in Canada is because there aren't enough of them, it is all the same problem).