The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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=63b12b46-3ad8-4696-9ca8-56a48a3e906d&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate