The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/publications/the-customs-tariff-rate-quotas-regulation?utm_source=06cc927a-fe8e-4d97-a83b-56f8505f0eb9&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate