Don't think DVLA maintain computers with VOSA data [MOT] or insurance info.
What I suspect happens is that the PNC has links to these different organisations all linked together by the car's index number [reg] so they can pull together a full picture [I've worked on multi-source IT systems like...
Ah, but what about all those petrol lawnmowers? They don't use the roads but if duty were added to fuel cost, they would effectively be indirectly contributing to road maintenance.