The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/publications/vat-for-businesses-if-theres-no-brexit-deal?utm_source=74e83c02-7d51-431f-904c-0bd360eb293a&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate