First of all, if you plan to maintain an EASA Class 2 medical, get an SPL, not an LAPL(S). The SPL allows you to be paid to instruct, the LAPL(S) does not.
Put the TMG on the SPL, it doesn't cost any extra so put it on.
The question that remains is do you want to apply for an aeroplane licence as well? If you don't, you will lose the right to fly EASA aeroplanes (excluding TMGs) as P1 after April 2015. You cannot fly SEP aeroplanes on a sailplane licence.
The current advice is that PPL licence conversions will continue beyond April 2015 so if you want to fly SEP again, eg be a tug pilot, you can convert from CAA -> EASA after April 2015. If you do, make sure that you add the TMG as well so that your TMG hours count towards revalidating the SEP rating.