Full information about transferring and retaining number plates can be found
on the gov.uk website:
If your number plate is on retention then completing the transfer is very straightforward. You need to be named as the grantee or purchaser on the document to have the right to transfer the number plate - see can I sell my number plate
The person purchasing your number plate will need to be added to the retention document as the nominee - this will allow you to transfer the plate to a vehicle registered in their name.
To add a nominee to your retention document you must complete the add/change nominee section and send the document to your nearest DVLA office- if you send it by post it take on average 10-12 days depending on how busy the DVLA are.
To transfer a number plate from retention to a vehicle is known as assigning a numberplate.
If you hold a number plate on retention then to assign it you need to transfer it to a vehicle registered in either your name or that of the nominee.
To assign the number plate you will need to send the following to the DVLA:
Often when selling number plates- the buyer is added as a nominee to the retention document to allow it to be transferred to a vehicle.
Once the document has returned, your buyer may now transfer the number plate to his/her vehicle, once the transfer has completed you have "exercised your rights" under the cherished transfer scheme and ownership / entitlement of the plate changes to the owner of the vehicle upon which the plate now sits.