I didn't say that you can't do the flight - he is perfectly entitled to display his skills to you. However, you cannot then sign the Certificate of Revalidation since this is specifically prohibited by Article 72.
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.