https://www.gov.uk/government/publications/notification-of-an-alteration-to-a-public-service-vehicle-vtp-5?utm_source=5adf845f-e52e-41b1-87dc-99eaf404ccea&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate
Provide a notification of alterations if you've made changes to the specification of a public service vehicle (PSV).
public service vehicletelldvsaalteration