Queen's Regulations Ch 5, Part 9, Para 5.366
"Hair:
Males. The hair of the head is to be kept well cut and trimmed, except where authority has been granted otherwise on religious grounds; style and colour (if not natural) is not to be of an exaggerated nature. If a moustache is worn, it is to be trimmed and not below the line of the lower lip. Beards and whiskers are only to be worn with authority, which will usually be granted only on medical or religious grounds, or where tradition permits. The appearance of the beard and whiskers is to be neat and tidy."