in terms of the regs I can understand why no beards, field hygiene etc. I've had a full set for 40 years since leaving, saved a fortune on razor blades so I've seen the fads come and go. Pony tails the future, very 18th Century in fact,
I 've always fancied a Frock coat and a rapier, pity they didn't issue us with cutlasses latterly.
As far as I can see the only reason for not allowing beards and long hair for men is discipline. If sailors, pioneers and pipers and certain religious groups can have beards then it demonstrates that there is no real practical reason. If women can have long hair then it demonstrates that there is no reason that men shouldn't be allowed to as well. Indeed it seems to be discriminatory.
I'm all in favour of discipline but it should be there for good reason. We expect soldiers to do what they are told to do. They/we should be able to trust we're being told to do something for a good reason. That trust breaks down when we're given arbitrary things to do for no reason.