The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/export-horses-and-ponies-special-rules?utm_source=d6f1f4cd-b15d-45d9-a2b6-53b3fdb0d1d4&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate