Thank you again for the reply.
Curiously, I have actually come across a couple of EASA licences (non-UK) that specify MPH for the instrument rating.
I'm not allowed to post images with my post count yet, but this is what is listed under the type ratings in XII (where your example just says Instrument - Nil):
Class/Type/IR - Remarks/Restrictions
IR (MPH) - IR Valid for MPH only
-or-
IR (MPH) - IR valid for MPA only