The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/import-fish-after-a-no-deal-brexit?utm_source=d284bc1d-cea2-4c2f-8613-a2fa10f1c378&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate