The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/accounting-if-theres-no-brexit-deal?utm_source=7ad3d003-4e7c-4368-8dc0-3d4e69a02f92&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate