The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/the-fisheries-sector-and-preparing-for-eu-exit?utm_source=be9c14c3-7a53-403b-a1c8-cc6166699cbd&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate