I've just looked this up, CAA Safety Sense leaflet 22 says the following:-
"the radio operator must also be in possession of a Flight Radiotelephony Operators Liceense (FRTOL). Glider pilots and student pilots under training are exempt from the requirement to hold a FRTOL. However, glider pilots without a FRTOL are confined to the use of air/ground communication using the nominated 'glider frequencies' ".
Interesting, so when a glider pilot without a radio license but, say carrying an ICOM, lands out at a GA airfield, they would be breaking the law by making circuit calls on the airfield frequency. Strikes me that there's something wrong there somewhere.
G