The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/uk-transition-communications-resources?utm_source=f5a54ab1-452b-4b70-b9b8-5ec855c76bbf&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate