Sorry, no idea - I did the NPPL SLMG route, back when TMG examiners were as rare as hen's teeth.
I can't believe that they would make you do a complete navex, that would surely be unecessary for a qualified pilot. The NPPL SLMG syllabus has a section on SLMG/TMG specific exercises, section 5 of this
document, that covers the main points; I would imagine the CAA TMG test covers the same areas plus the usual circuits, field landing, EFATO stuff.
The only time I flew with a TMG examiner was when I managed to let my SEP & TMG ratings expire, and had to revalidate both by proficiency check. He was a very nice chap, had ratings for this, that and the other and eye wateringly expensive fees to match.
My knowledge is very limited, but I don't know of anywhere that offers actual TMG training.
Oxfordshire Sport Flying are well known SLMG specialists, many of the larger gliding clubs can offer SLMG training, I would be interested to hear of a powered school/club that has TMG instructors, examiners & aircraft.