Can't see why an FI who is temp without medical couldn't supervise
Then try reading Article 72. Unless granted a written exemption by the CAA they cannot exercise the privileges of the licence or any rating contained in it! Supervision is one of those privileges.
Requirement for a medical certificate
72 (1) This article applies to any licence granted under article 64, other than a National
Private Pilot's Licence (Aeroplanes) or a Flight Radiotelephony Operator's Licence.
(2) The holder of a licence to which this article applies is not entitled to perform any of
the functions to which the licence relates unless the licence includes a valid medical
certificate issued under paragraph (4).