Yeah, Lottie is spot on - if the origination of a large part of the car cannot be confirmed, then a Q plate is issued.
It used to automatic for a kit car, but then they changed the rules to a points based system - and providing sufficient parts of an original vehicle (of known origin) were re-used then that identity could be carried forwards. Different points for different bits of car.
Hence my Speedster kit car isn't a Q plate (it's a 1971 J reg) because enough of the Beetle it is based on - YDP 411J - was reused.