I've never flown a motor glider, only the ordinary kind, so I'm not sure what level of instrumentation they have.
Are they equipped to cover the instrument and radio nav parts of the PPL syllabus? If not, that's probably part of the reason you can't get a GrpA PPL on them, as well as the requirements for public C of A etc.
You might be able to save money by doing the PPL first and then cross-crediting to the NPPL with SLMG rating.