That is why it is stupid and doesn't work! The greatest percentage of under-occupiers are age 61+. They are all exempted, even though many that age might possibly be still in work. And, on top of that, in a great number of councils, the lack of places for them to move.
And the suggestion that they go to private housing. LOL. That's going to save money? Even with caps, the housing benefit bill rises, it already has. Because the problem isn't entirely under-occupying. It is that the buy-to-let and system of buying property here has over-inflated housing prices and consequently, rents, on top of the 'No DSS, no children, etc.'
You move a 60-year-old still in work, not claiming housing benefit on his under-occupied 3-bed council flat to a 1-bed private let flat and he needs to claim housing benefit, even though working. And this is assuming he can get a private let as he needs 'DSS'.
The whole thing is such a stupid exercise in putting a cart before a horse and bolting a stable door after the horse has left it's a cruel joke on those in the worst of situations.
But of course, you'd actually have to have knowledge of how things really are.
We can't expect that of people who can't be arsed to figure out who gasses children to death in their beds, can we?