An Arcus M is indeed magnificent, but never a TMG.
The UK's definition is the EU one:
‘Touring Motor Glider’ (TMG) means a specific class of powered sailplane having an integrally mounted, non-retractable engine and a non-retractable propeller. It shall be capable of taking off and climbing under its own power according to its flight manual.
(From the Aircrew Regulation Reg(EU) 1178/2011)