The HSE article covers systems that go negative when the wearer breaths in, risking contaminants being drawn in. An aircrew mask has flex seals that use the positive pressure provided by the system to push against the face and ensure that any leaks are outboard - ie the complete reverse of the HSE article.
AvMed have been happy with beards if you can achieve a suitable seal when tested. One presumes that any court case would bring in AvMed experience rather than an HSE document that may be used by a painter and decorator. If you pass the routine check fit with your mask / helmet / headset then all should be good for the DH.
Incidentally, given the care, selective fitting and routine check fits most of us are used to then the civilian side where either a shovel or alien mask with a one-size-fits-nobody rating stuffed in a cubby as part of the basic aircraft fit looks a little more cavalier!