Current UK Absolute Height Record for a glider was set in 1995 and is 37,959' (11570 metres) in a DG 500 glider.
The world record (ratified so pre-dates the current claim) from 2006 is 50,721' (15,460 m) in a DG505 glider.
My glider's VNE (IAS) decreases from 151 kts at surface to 97 kts at 32,808' (10,000 m). I G stall occurs at around 37 kts IAS. Figures for greater altitudes not provided.