MGIR & SLMG instructor ratings are not the same thing.
A BGA MG instructor cannot instruct for the purposes of the NPPL & you are unlikely to come across one operating outside of a BGA club. SLMG instructors ratings may be administered by the BGA but are recognised by the CAA.
What I don't know is whether NPPL SLMG training hours will count towards an EASA LAPL(A) licence if a pilot chooses to add an SSEA before converting to EASA. Even if they don't, an EASA LAPL TMG can easily be turned into an LAPL(A).
Since the OP is talking about doing a JAR TMG, the whole NPPL thing is moot.