Gliders in wave do climb to over 14,000 in Wales. In Scotland, I climbed to 20,300 agl.
But once in the US, flying an Arrow, with a teenage smoker as passenger, the Naval Air Station asked if I could accept 12,000 feet to avoid some fighter exercises. I told them I was fine with that, then told the teenager to let me know if his fingernails were turning blue; as a smoker he would be less able to tolerate altitude. He spent the rest of the flight furtively examining his nails.