The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/exporting-and-importing-fish-if-theres-no-brexit-deal?utm_source=4d83f92f-27f3-4cfe-b558-91b40b3c0392&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate