Even though the car is on the VW system, it will state clearly on there that it's an import. It will say PI) before the model and spec. Also, I wouldn't expect many people to privately buy a GTI without HPI checking it first where it will become clear (either by saying so or the fact that half the info is unknown). A dealer always HPI checks a car before valuing it. Main dealers are also legally obliged to point out to a potential buyer that the car is an import.