Surely a lot of this is down to the reduction in payments to local authorities from central govt. There was a sharp reduction in the govt grant in 2010 (38%). Since then there have been many changes to local authority funding, such as changes in the way in which business rates are allocated, freezes to rate increases etc etc. Local authorities in the UK are not allowed to borrow. And if you remember, when the right to buy came into force under Thatcher, the LAs were not allowed to invest the proceeds in building new homes.
The LA housing stocks, in many cases, were hived of to housing associations, some of which were formed by the housing department staff. Now, many of the original associations have been subsumed by larger organisations which, to my mind, no longer have the original ethos of providing secure social housing.