Full information about transferring and retaining number plates can be found
on the gov.uk website:
If your number plate is currently held on your vehicle then to transfer it from your car to that of the buyer is very straightforward.
You will both need to complete and sign the DVLA form V317 for both the "donor vehicle" (your car) and the "receiving vehicle" (their car)
When you transfer a number plate from vehicle to vehicle- you will need to send the following to the DVLA
To transfer a number plate from vehicle to vehicle costs £80 at the time of writing.
To transfer a number plate from your vehicle to a retention certificate you will need to complete form V778/1.
You must be the registered keeper of the vehicle to be "entitled" to the numberplate.
Simply complete form V778/1 and send to the DVLA along with
Once you have sent off these documents- it can take up to 4 weeks for the transfer to complete.
Once the transfer is complete you will be issued with new tax discs for both the number plate you transferred and the "original" reg number which is then applied to the donor vehicle.