"Nothing is ring-fenced for anything. So the TV licence fee is not guaranteed to go straight to the BBC, in the same way that Road tax/Fuel Duty isn't put aside to repair/build roads (road repair comes out of the money allocated to councils, which is why the roads are always needing repair)."
This is just not true.
The BBC's charter says
"
(1) The Secretary of State must pay to the BBC out of money provided by Parliament sums equal to the whole of the net Licence Revenue or such lesser sums as the Secretary of State may, with the consent of the Treasury, determine. "
[in practice such 'lesser sums' are very hard fought by the BBC, and are strictly limited - the government can't just go say 'we need to buy this breast cancer drug', and cut £500 million from the BBC, everything is agreed and fully negotiated within very strict and limited parameters - the BBC get almost all of it]
' “net Licence Revenue” means the amounts paid by the BBC into the Consolidated Fund under section 365 of the Communications Act 2003, less the expenses incurred by or on behalf of the Secretary of State in relation to the administration of the television licensing system. '
So it IS ring-fenced, and the BBC collect the money, which means that if the BBC's scummy salespeople collect £5 billion, the BBC receive £4.7 billion, if they collect £5.2 billion, they receive £4.9 billion and so on.
That's just not true of car tax or other things.