There's certainly a regulation turban for our Sikh chaps. Back in my UAS QFI-ing days at RAF Abingdon, we had a Sikh lad and I had great fun insisting that stores provided him with one...
"A what, Sir?"
"RAF pattern turban, Sikhs for the use of!"
"Isn't such a thing!"
"Is - get one please!"
So they did - but by the time it arrived the boss had read him his tea leaves at the 6-monthly 'Night of the Long Knives' (which he'd introduced to weed out students who had poor Progress/Attitude/Technique scores) and he was no longer with us. I've no idea how he used to do so, but he managed to get his hair and 'sleeping turban' under a normal bone dome.
No doubt there's still a single RAF turban on the stores shelf at 'Dalton Barracks'...