Aerobatic Flyer - I've either missed something or you're making this much too complicated:
You have a UK PPL with a SEP class rating
You want to be able to fly TMG and SLMG
All you need to do is pass a skill test with a class rating examiner in a TMG (like the Grob). The CAA will give you a list of suitably qualified examiners if you ask.
I have a UK licence and that's how I got a TMG rating put in it.