Copy and paste from the CAA Doc 24 (examiner standards):
1.14.3 Applicant’s Medical Certificate expired or absent Where the applicant for the LPC has a valid licence but an expired, missing or suspended medical certificate, the test may be conducted (in a simulator only). If successful, the Certificate of Revalidation should be signed in the normal manner. The applicant shall be told that they cannot exercise the privileges of that rating until they have a valid medical.