Quite often the one that was on it before it was registered as private. I've changed car a few times and took my plate with me and the car was always given the one that it had on before mine.If not it has to be re-registered with a new registration.
It's also the case in foreign registered vehicles - they can be re-registered with a vacant UK number.
Just make sure you transfer the number BEFORE you sell, or the number becomes the new owners property.
DVLA offices don't do the transfer while you wait anymore it takes 10 days ish.