The gliders from at least one of the sites alphaalpha mentioned, Ridgewell (where I fly) also go right up to the Stansted CAS boundary - we don't just stay overhead our aerodrome. If we go cross-country West or North West from Ridgewell we skirt the CAS boundary via Haverhill, Linton etc. until we can follow a straighter line to a turn point (usually before returning to Ridgewell - most glider XC's are intended to be closed circuit routes).
On one occasion I was over Haverhill, thermalling, monitoring Essex Radar, and heard them working a power plane round the same edge towards me. They advised him gliders were operating, he replied that he could see none. I called up then, intending to advise my height and position to help avoid conflict, and ATC just gruffly replied "Glider XXX stay outside CAS". End of conversation.
I know ATC can be very busy, but if they can find time to talk to power planes skirting their airspace, they might at least offer the same courtesy to gliders instead of refusing to talk to us at all.