From Standards Document 3 :
http://www.caa.co.uk/docs/33/SRG_FCL_03_A.PDF
Applicants must be in possession of a JAA Class 1 medical certificate at the time of the test. The medical certificate shall be shown to the examiner. If the certificate is out of date the examiner may still conduct the test, but the applicant should be aware that, regardless of the outcome, he shall not be permitted to use his licence or rating until the certificate is revalidated. UK armed forces personnel must hold a valid and current, military aircrew medical category.