This is not within the gift of the CAA but is clearly laid down in JAR-FCL 1.155 -
(b) Modular Course
(2) Crediting. From the 200 hours of flight time:
(iii) 30 hours as pilot-in-command in touring motor gliders or gliders.
If the CAA refuse to accept glider hours, quote the relevant JAR and threaten them with an appeal under Regulation 6 of the Civil Aviation Regulations 1991. That usually makes them cave in.