From LASORS:
In the UK for licencing purposes there is no destinction
between SLMG and TMG aircraft. UK issued licence
holders with a valid SLMG or TMG rating can fly any
motor glider (SLMG and TMG).
SLMG:
‘an aircraft with the characteristics of a non power
driven glider and which is fitted with one or more power
units which is designed or intended to take off under its
own power’
TMG:
‘a motor glider having a certificate of airworthiness
issued or accepted by a JAA Member State having an
integrally mounted, non retractable engine and a non
retractable propeller plus those listed in Appendix 1 to
JAR-FCL 1.215.'
The main advantage of holding a TMG Rating for pilots already holding SEP Class Ratings is that those who fly both SEP Class and TMG Class aircraft can count time on either towards revalidation requirements for both - which could be substantially cheaper for them. Whereas without such a TMG Rating, although the holder of a UK JAR-FCL licence with SEP rating may, subject to differences training, exercise the privileges of their licence on SLMGs, they may not count hours flown under national SLMG privileges (even in an aircraft which meets the JAR definition of TMG) towards the re-validation of the SEP Class rating.
The BGA are the guys to speak to about flying SLMGs/TMGs. But to get the maximum benefit, you would need to add the TMG rating to your PPL.