The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/collections/brexit-guidance-for-dcms-sectors?utm_source=a3072f55-7f90-4802-a324-e6b5e0d4d86f&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate