The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/visiting-the-uk-after-brexit?utm_source=35b10156-1d06-4c42-81c4-b031c0de9357&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate