Full information about transferring and retaining number plates can be found
on the gov.uk website:
Retention is a scheme which enables you to hold a number plate "on certificate" and means you don't need a vehicle if you want to buy a personalised number plate.
The retention scheme is useful for the sale and transfer of number plates as it clearly states who owns the number plate in question and provides proof of its particulars.
To retain a number plate you will need to complete form V778/1.
You must be the registered keeper of the vehicle to be "entitled" to retain the numberplate.
Simply complete form V778/1 and send to the DVLA along with
When a number plate is held on retention- the rights to the number plate expire after 12 months- this period of time is the entitlement period.
To transfer the number plate to a car or to extend the entitlement- the expiry date must not have passed- as soon as the entitlement period ends- you lose any rights to that registration number.
On the retention document you may sign and date the "extension of entitlement" period box and send this to the DVLA Swansea along with a cheque for £25 payable to "DVLA SWANSEA"
Read more here about DVLA fees