I've climbed to 12,000' in East Anglia a couple of times. Slightly more challenging as I did it both times in a glider in cloud. Climb rate was measured at 1,000' per minute and it took about ten minutes once in the cloud.
Climbed to 27,500' in a glider in the States along a wave bar over Minden (Nevada). Was still getting a climb rate of around 200' per minute but didn't go any higher due to my oxygen consumption and it's a bit chilly up there!