The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/grants-for-businesses-that-complete-customs-declarations?utm_source=d34b729b-9f10-47a7-8c26-787e8b170864&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate