The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/news/brexit-workshops-get-small-businesses-ready-to-trade?utm_source=a1eb8d08-83bc-402a-8713-1a7c4cb244ef&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate