Although some of the bigger gliding clubs may be listening out on the radio if they are expecting a visitor, (prior permission absolutely essential) flying overhead is dangerous, a radio call will not keep you legal or safe. The people listening to the radio are not running an ATC service, and can give no instructions or clearances. An encounter over a gliding site with the braided steel cable used to launch gliders up to 3,000' agl is not desirable.
A collision between a glider and a power plane is more likely to be grief for those trusting in engines. Most recently,an RAF Tutor doing aeros in a crowded corridor on a good soaring day flew into a glider from below. The two on the RAF Tutor died. The glider pilot parachuted to safety.
A while ago, a K13 glider not far from Aylesbury/Thame was struck from behind, about two feet of the wing was sliced off. The power plane crashed and the occupants did not survive. The K13 glider was able to land safely in a field.
Gliding sites are well marked on your charts and should be avoided. The advice to plan your flight well above cloudbase should give a safer and smoother ride