(I'm assuming you mean NPPL, not LAPL.) Have at look at licence allowances from the
NPPL website. You will see that BGA GPL holders get a large reduction in minimum training hours for both NPPL(SSEA) & NPPL(SLMG).
Fromthis SLMG I know after 75 hours you can get it upgraded to a TMG rating.
Ignore this, it's a red herring. SLMG is the CAA term for what EASA calls a TMG; when you apply for an EASA licence, SLMG will become TMG.
Yes, you can tug in a TMG with an NPPL(SLMG.)
TBH, with that many SSEA hours in your logbook I would just do the SSEA skills test & add an SLMG later. 10 hours of SLMG training + the skills test will cost £1000 to £1500, depending on the aircraft & location.