The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/government/news/eu-exit-guidance-for-agri-food-stakeholders-from-defra?utm_source=b484a867-666e-476d-a552-a44678452f59&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate