It is still definitely what they tell one in the forces though, so if it is an urban myth indeed, it is now so well ingrained in thinking there, they've lost track of the fact then long ago for sure! Only folks I knew of allowed a beard, by long tradition, in the forces were certain higher non-officer ratings in the RN!And I do not think the ruling applies merely to a days stubble, but rather more to a 'full' grown beard here though, in any case there anyways.
Every male in the RN can grow a "full set" by requesting to discontinue shaving. As for, "higher non-officer ratings", WTF are those? There are junior ratings (ordinary, able and leading ratings), senior ratings (petty officers and chief petty officers) and warrant officers, all non commissioned. Then there are cadets, or upper yardsmen, and midshipmen (pre commission trainee officers). The officer class all hold the Queen's commission.
For what it's worth, when I enlisted we were admonished never to pronounce lieutenant, leftenant on pain of extra drill, instead we were instructed to pronounce it l'tenant. We were also exhorted to pronounce madame or ma'am, mum. These pronunciations have now gone by the board, I believe.