Rapid antigen tests can only serve as a reference and cannot replace the nucleic acid test which is at present the gold standard for diagnosis of COVID-19. Notwithstanding its limitations, rapid antigen tests may play a role in facilitating access to testing and earlier diagnosis in some people.
From CX webpage ,
documentary proof of laboratory or healthcare institution “can be in the form of copy of ‘Certification of Accreditation’, ‘Certificate of Compliance’, or information printed out from an official government website”.
By clicking the link for UK shown in CX webpage , we found that ExpressTest is on the list of general COVID-19 testing providers updated by the Department of Health