The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/check-temporary-rates-of-customs-duty-on-imports-after-eu-exit?utm_source=d774c866-5708-4f24-9739-e4e013f306b1&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate