The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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?utm_source=cd86bf19-0b29-429a-a8c7-8d22d965ca8c&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate