As for SLMG, the CAA have produced guidance which states that they interpret the regulations such that a JAR-FCL SEP rating holder may fly a SLMG in UK airspace, once they have completed differences training, using a valid SEP rating. In the same way, you could telephone or e-mail the CAA and receive the same guidance.
If this is their intention, then they must issue an Exemption to the ANO in accordance with Article 242; they cannot issue guidance to change the Law! In fact they did issue an Exemption ORS4 No: 674 when it was previously pointed out that they had cocked it up! That exemption was withdrawn on 30 June 2009 leaving the statement in LASORS as an indication of their continuing ineptitude. Much of this results from someone at the CAA agreeing to things produced by the NPPL committee, and then failing to tell anyone else about it, least of all the lawyers who produce the legislation.
As I see it, you are an examiner acting on behalf of the CAA in accordance with the Authorisation granted by them
PPL Examiners have received little or no formal guidance from the CAA in over 12 years, they can only operate in accordance with the ANO, and not a bunch of inconsistent, badly worded, and inaccurate guidance documents.
Nothing in the Law prohibits an examiner from signing a licence with a Cof R whether or not the candidate has a valid medical certificate. If they do not have one you must make the holder aware that they may not exercise the privileges without such a Certificate. The CAA are not empowered to change the law by making it more stringent; they may relax it by issuing an Exemption under Art 242.