I think the easiest approach is for him to get a retention certificate for the vehicle. I think that costs £80. That will also issue a new number for the car which you then get plates made up for.
He can then sell on the retention certificate or put it on another car.
All this can be done via the DVLA web site.
Bloody hell you guys are on the ball, sat at home doing all this lot electronically. The first time I did the cherished number stunt I had to take the car to the nearest motor taxation office, stand in the rain whilst some minion decided that the thing in front of him was indeed a car; that it was blue & the long number on the damp paperwork was the same as the rusting runes on the bulkhead. 45 minutes & a drenching later he was happy & went back to his Lemsip, whilst a colleague relieved me of a not inconsiderable amount of cash for the day, £25 as I recall, however it was a long time ago. I’ve still got the number & it’s graced a number of cars.
I'm ok with online, my question was really about whether I could then transfer ownership of the retained plates to another person. I assume so or people couldn't sel l them without cars :
Once you receive confirmation that you are the registered keeper then the plate owner can remove the plate to retention
the only way to lose a reg is if you get caught misrepresenting it and it's revoked.
All he has to do is either put it on retention or put it on his new car.
Either way it costs £80.