Thank you, PropSwinger, for posting the link to the British Gliding Association listing of letters of agreement for flying in ye upper air spaces.
We do it on a regular basis, but under strict conditions, enforced by the club that has the permission, to comply with our concessions. And we generally use oxygen, above 10,000 feet....definitely above 12,000 feet anyhow! Some of us manage to qualify for the gold or the diamond gain of height above the low point at which the tug plane relases us, or above the low point you descend to while scrabbling to stay up at all at all....my personal best was 20,300 above a release height of about 3,000 feet from Deeside Gliding Club near Aboyne, in Scotland.
I had the pleasure of looking down on three bodies of water from that altitude,
the Murray Firth, the Firth of Forth, and the North Sea! The weather was sunny, but not warm, as I was trolling back and forth along the wave, the canopy frosted up on the shady side, and a banana in the pocket of the glider became a banana popsicle! The flight lasted slightly less than two hours, so the conditions were pretty good.
If anyone reading this would like to try for altitude by wave flying, I can offer a few tips. Gliders work better than power planes at doing this, by the way.