In its Notice to Stakeholders regarding Brexit, The European Commission was quite clear,
"Certificates issued before the withdrawal date by the competent authorities of the United Kingdom on the basis of the provisions of the Basic Regulation and its implementing rules will no longer be valid as of the withdrawal date in the EU."
Therefore, in a 'no-deal' scenario, a UK-issued EASA medical certificate will not be valid in the EU after 29 March. It will, of course, continue to be valid as a UK medical certificate.