Rare cars you dont see anymore,

[procta]

You know q-plates are just for exactly that right? Ie cars built from scratch...

 
Nope, I used kit cars as an example, as cars get Q plates for other reasons.

isn't it they can't establish the country of origin?

your close,  they can't establish the age of the car.

Close.  It's usually where the age or identity of the vehicle cannot be determined, often stolen/recovered where the original VIN etc have been removed and the original registration unknown.

They're often on kit cars because too many used/reconditioned major parts have been used (you can have one IIRC) to get a new age-related registration.  It's perfectly possible to do a kit car and not get a Q-plate though.

Its sort of a black and white area with kits cars, depending the kit car and also  how much the original donor car is used. 
 Even if you drop an engine in the back of a car, rather than having it at the front. That can cause a Q plate, as the identity of the car has been altered from the manufactures design.
It goes on a points system, you will have to look it up, as it covers stacks of stuff on it.
 Really does go into some right detail.