Normally, the ONLY British soldier allowed a "full set" beard would be a Sergeant in the Pioneer Corps ( grave-diggers !).However, most soldiers in both WW1 and 2 were denied the relief of a Shower,shave,and shampoo and became very hirsute and piratical after weeks or even months in the trenches.The defence of Tobruk is a good example.
A few years ago , on British TV, there was a version of "Moll Flanders", featuring a scene in which Moll is sitting below decks with the Captain of a 17th Century sailing ship ,aboard which she is about to take passage to the New World. She says to the Captain "Aren't you wanted on the bridge , or something ?" I was dumbfounded !