As an update to this, I just learned through a BBC programme on another subject that there are, wait for it, 4,500 derelict & unoccupied houses in the borough of Portsmouth alone.
That is an astonishing and quite disgraceful figure if one considers this to be a depressed area following the winding down if the Naval Dockyards.
Perhaps if you multiply this figure by the number of cities in the
UK you can begin to imagine the bigger picture, bearing in mind these properties are all owned by somebody.