The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/statutory-instruments-relating-to-eu-exit?utm_source=969d0a42-a67e-43b7-9d8e-b913aa542028&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate