The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/uk-trade-agreements-with-non-eu-countries-in-a-no-deal-brexit?utm_source=74c53612-9aac-4a27-987c-86a2d5ca9164&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate