The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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=d4a790b3-d778-480c-ada4-ad23cab72929&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate