Won't the sailplane & aeroplane licences be separate, as different categories of aircraft?
Perhaps so, however, EASA rules require all of a person's licenses and ratings to be issued by the same competent authority (FCL.015) so I don't see how the BGA can issue a licence because of that, however, they may go down the same route as the NPPL already has whereby they (the NPPL people) make a recommendation for licence issue to the CAA who then do it at a lower rate because the checking has been done. The CAA licence fees allow for a lower fee of £39 in that case - but the person making the recommendations has to pay up to £5,680 a year for the privilege so may also want to make a charge.