The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/trading-with-developing-nations-from-1-january-2021?utm_source=b1f37ef9-dd2c-4060-84b9-6d1fb170f484&utm_medium=email&utm_campaign=govuk-notifications&utm_content=daily