From CAP804:
(d) Crediting. Applicants holding a pilot licence for another category of aircraft, with
the exception of balloons, shall be credited with 10 % of their total flight time as PIC on such aircraft up to a maximum of 10 hours. The amount of credit given shall in any case not include the requirements in (a)(2).
The problem is that to date glider pilots do not have pilots licences. This is shortly to be changed under EASA