Road tax (I believe it's actually properly called Vehicle Excise Duty) has never been hypothecated for road repairs or maintenance.
All tax revenue in the UK goes into a single pot and the treasury distributes it as per the Government's spending plan. Even National Insurance contributions aren't earmarked for the NHS.
The numbers of pre-1973 cars regularly in use on the roads is very small, and the mileage they do is pretty low in comparison to modern cars. The settings of VED rates are classic examples of 'nudge' theory in action, encouraging people to select cars with lower overall emissions.
