Q plater are usually reshelled cars I think, or a certain percent of the original vehicle (certain items) has been replaced.

May also be that for some reason the original id cannot be put back on the car, ie stolen and all the id has been cut out, so can't be replaced.

Always worth a chunk less than a normal example, and will be harder to sell.

If it was a car that you are going to keep, is in good condition and cheap then it's ok, and you don't mind the Q