Will I be able to teach on a TMG also? I know the CAA often view these as one and the same?
So long as you have the TMG Class rating in your licence Yes. They do not consider them the same thing, they are different Classes however; they can be mutually revalidated.
FCL.905.FI FI — Privileges and conditions
The privileges of an FI are to conduct flight instruction for the issue, revalidation or renewal of:
(a) a PPL, SPL, BPL and LAPL in the appropriate aircraft category;
(b) class and type ratings for single-pilot, single-engine aircraft, except for single-pilot high performance complex aeroplanes; class and group extensions for balloons and class extensions for sailplanes;
Your biggest problem is finding a school that teaches TMG.