The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.
https://www.gov.uk/guidance/prepare-to-drive-in-the-eu-after-brexit-bus-and-coach-drivers?utm_source=cd175009-5bf3-4b70-a58a-3eaaf4d5d0fe&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate