The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/run-international-bus-or-coach-services-after-brexit?utm_source=1c38e48d-dc86-4d3a-852b-d001938221f4&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate