If you have a TMG rating then you already have a JAA PPL. To add a SEP Class rating to that PPL you do sufficient training to pass the SEP Class Rating Test. All quite simple.
If you have a SLMG PPL then that is now a NPPL and you will have to do lots more to get your JAA PPL with a SEP Class Rating i.e. all of the exams, 30 hours training and an initial issue Skill Test.
There was a SLMG + 75 hours conversion for pre JAR pilots holding a UK PPL but that won't apply to the NPPL.
If you have a SLMG NPPL to add a SEP Class to the NPPL you just do differences training. This is the easiest and cheapest option but won't get you a JAA PPL.
If you want to spend a lot of your future flying, the JAA PPL is the only route. A TMG rating will not be any cheaper than a SEP rating.