G-VTUS Won't be using thermals, they simply don't rise that high. He will have long since tucked the engine back into the fuselage. He will now be using mountain wave lift that goes up above airliner height. The UK altitude record for a glider in Scottish mountain wave is 38,600 feet. The world record for a glider is 50,671 ft in New Zealand mountain wave lift.
You have to remember the cockpits of gliders are unheated and the outside air temperature at 17,000 is usually between -10 to -15 Centigrade, and at 38,000 will be -50 to -60 Centigrade.
Kudos