The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/coronavirus-how-to-help-safely--2?utm_source=1e8a0e2e-5cc1-4650-b336-74410037e4dc&utm_medium=email&utm_campaign=govuk-notifications&utm_content=daily