RAF allows moustaches too. Apparently "not to extend below the edge of the mouth" which still leaves quite a lot of room for expression I would guess. Rule still applies while on secondment [1].

My Grandfather was born before the days of safety razors and tells me that Navy personnel were able to petition to avoid shaving on account of delicate skin (something about the salt water and fresh air). I think anybody who could grow a beard would have done.

1. http://news.bbc.co.uk/1/hi/scotland/7451939.stm

Interesting observation. The area of the upper lip is known as the "Danger triangle" of the face, in which due to way blood circulates in that area, including the upper lip, infections can spread back towards the brain:

https://en.wikipedia.org/wiki/Danger_triangle_of_the_face

I never thought that mustaches had a health / self preservation / natural selection origin, but it could very well have been the case. Shaving every day without modern tools and products could easily cause infections, which in that area could prove fatal.

What about trimming to a stubble, was that ok? Or does it have to be razor blade shaven?

It will seal just fine up to a 4 or 5 inches or more I can't remember anymore. I do remember it being discussed when I went for training to be the company NBC NCO. And it boiled down to appearances more than anything.

Also special forces are expensive to replace and are out in areas that could have NBC related threats and have some of the biggest beards I've ever seen.

A gas mask seal will not be great with significant stubble.