RallyChris

The age of the donor car will dictate the age related registration that the DVLA issues you with for the kit car, assuming you have used enough of a 'single donor' to qualify. In the old days (certainly SVA era) you could retain the original donor car's registration number but that gave rise to the large number of incorrectly registered and non tested kit cars that are regularly seen today.

Of course, if you aren't using a single donor the car will get a Q plate anyway.