The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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-of-products-animals-food-and-feed-system?utm_source=982043a6-a5da-4524-b6b0-4a9efab787f7&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate