The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/identify-livestock-for-export-to-the-eu-in-a-no-deal-brexit?utm_source=8c321fbd-ac47-40d5-a17b-90a9d26c3906&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate