The end of ye olde Tax disc..

[Grant]

Correct. It goes into the central treasury fund.

But where does the money come from to fix the roads?

Local council funding which yes is influenced by that central pot but only a set amount and not by how often cars use the road.

Fair point.

UK motorists paid £5bn in road tact last year. The government (apparently (you wouldn't believe it driving around here!)) spend £9.5bn on upkeep of the roads. Arguably, it would give more money to the 'fund' to allow increased spending. But thats an un-educated guess.