they could withdraw the reg mark but that would not change the vehicles i.d. so the vehicle would get an age related plate. 'Q' plates are not issued for this purpose.

here's what the dvla say about the 'q' plate....

What is a 'Q' registration number?
These are a very useful consumer protection aid.

A 'Q' number is a clear indicator to a prospective purchaser that the age or identity of a vehicle is unknown. The vehicle may be rebuilt from parts, some or all of which may not be new. This also applies to vehicles imported without supporting evidence to identify the vehicles age.