Oh boy...
I'm old enough to have been in the forces when the current KC and also PA were either serving or did extended training courses ( RAF/RN), such as flying training etc.
As far as I know they both seemed to try and comply with the appropriate regs for the appropriate service with regard to uniform, beards etc. (Navy beards OK, RAF not)
Personally in more modern times I'd still expect that PW would comply with the rules in a similar manner and I know pre-2019 many forces personnel ( and we're talking commissioned and non- commissioned) would have raised eyebrows at PW if he'd have turned up at an RAF station in full RAF uniform to do a lengthy course sporting a beard.........
Now of course PW might have "gone stuff it, I don't care, I'm a Prince"...or just maybe PW understood the idea of leadership by example....if so good on him.
If the Queen was involved at all in all this it might have been a comment along the lines of "I'm not sure about the beard, what do my regulations say".............🤔