Central government capping of elected local authorities would be unconscionable in any advanced democracy. And so it was in the pre-Thatcher days when this country was in fact an advanced democracy. The right to cap an elected local authority is the right of its electors at the ballot box.
But Police Authorities are now a different matter. If they were properly part of local government, or even if we had elected sheriffs, then they might have cause for complaint. But they can't have at the moment.