All cherished and private number plates are subject to a dvla transfer fee. The buyer and seller both have to pay a transfer fee. If the number plate is on a vehicle at the time of sale, the plate can either be transfered directly to another vehicle providing both vehicles are taxed and mot'd. This dvla transfer fee for this will cost £80.
If the buyer requires the number plate on a retention document the plate will have to be taken off the vehicle via the dvla and the cost for this transfer is £105, again the vehicle has to be taxed and mot's in order to do this.
If the buyer does not have the vehicle ready for the number plate to go on a retention document will be required as you are able to retain the number plate for up to a year, and then renew the document each year after that for a further £25 to dvla. Therefore you are able to retain a number plate on a retention document for as long as you like.
If a private number plate has been bought directly from the dvla and never been placed on a vehicle it will be held on a Certificate of Entitlement, if a buyer requires a plate that is on a Certificate of Entitlement to go on to a vehicle a nominee change will be required unless a car to car transfer is taking place.