2)!london is not monolithic; it has several local councils all with differing rules. They'd all need to agree and coordinate. I don't know why it isn't done better.
3) people living in slums ether don't know their rights; or how to enforce those rights. Sometimes their own legal status is dubious and they risk deportation. Even if they do know their rights, and how to enforce their right, and they can get the regulator to take action, and they're totally legal and above board, they may just end up without a home.
Housing in the UK is weird and broken and at the low end there are some strong weirdnesses built into the system.