Noise Unit
Fiction, as far as UK registered aircraft are concerned.
There is a requirement for a suitable quick-donning mask to be readily available.
On my last aircraft type I spent many happy hours substantially above 40,000 ft and never once wore a mask.
Notso
I wonder if you missed part of av8boy's post?
...US Regs:..14 CFR §91.211…Supplemental oxygen…(a) General.
No person may operate a civil aircraft of U.S. registry...
Those particular regulations are not binding on UK registered aircraft, so you (and I) don't have to follow them.
Regards
Bellerophon