The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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=51a9f961-4618-494b-a63a-182bce0057d3&utm_medium=email&utm_campaign=govuk-notifications&utm_content=daily