Take a registration number off a vehicle online
https://www.personalisedvehicleregistration.service.gov.uk/retain

Put a registration number on a vehicle online
https://www.gov.uk/put-registration-number-vehicle

V317 Application to transfer or retain a vehicle registration number
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/409911/V317_211114.pdf

V62 Application for a vehicle registration V5c Document certificate
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/471730/V62_030815.pdf