I think BTL needs to be made next to completely unprofitable. So much so that your average man on the street isn't going to see it as in any way viable. Even loss-making.
This isn't to stop the average man on the street making money - just not through providing what should be a basic human right.
Let housing associations and the local Council provide rental accommodation on a non-profit basis.
And increase our rather pathetic regulation around rental and tenant's rights, which is in no way comparable to our European neighbours.
e: There may need to be some mechanism to allow private rental of high-end, luxury properties, which nobody actually needs to live in. But your average terraced house where most of the tenants work in care homes or the local supermarket, those shouldn't be profit making vehicles at the expense of the working classes.
OK, that's BTL. But what about my circumstance? If it cost me money then I'd just keep it empty which would further the issue and consider it a retirement fund or an asset to sell when I needed the money.