The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/guidance/manage-your-trusts-registration-service?utm_source=8c4487c8-a566-4dd3-bc8b-2b4a9b1b06f8&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate