The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/hmrc-brexit-communications-resources?utm_source=53f1ccd9-567c-48a3-b685-e76879c76b5b&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate