The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/making-declarations-using-transitional-simplified-procedures?utm_source=23279972-2a0a-4006-9cce-6aff194c2dcb&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate