To my knowledge there are no rules against it, it is just a major and expensive plumbing project requiring redoing the whole kitchen. And so as a landlord why bother, you will always find someone willing to rent, no matter how unlivable the place. This dynamic was hardly unique to the Barbican, it was the reality of being a tenant in London, and ultimately one of the reasons I left. London's housing stock is just terrible compared to every other city I've lived in.