You no longer want to assign the registration number.
You can apply for a refund of the assignment fee (if you paid the assignment fee at the time you purchased or retained the registration number).
If you have a certificate valid for 10 years, you can apply for a refund before the expiry date of your V750 or V778 certificate.
For any other certificate, you can apply for a refund within six years of the expiry date.
Once a certificate has expired, you lose the right to display the registration number.