The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/claim-vat-refunds-after-brexit-if-youre-an-eu-business?utm_source=e4d16eef-1817-40db-b4e7-435817357b1c&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate