Finish your NPPL SSEA, then add, via a conversion flight, the SLMG rating to the NPPL.
Once you have that, convert to the LAPL(A), with a TMG endorsement.
With that in your hand, convert your BGA Glider Pilot Licence to an LAPL(S), again, with the TMG endorsement.
Provided you do the hours etc in the TMG, you will be able to keep it current
on both licenses.
Thats my understanding at least.
Cheers