For a thread that anticipated this discussion by several months, see 'Do we need armed sky marshalls' on Questions.
I must admit, several of the contributions are from me - I saw an job advert on the back of a magazine while I was signing on. The ad didn't explicitly say 'must have been member of the SAS' (mind you, there are quite a few units in the British Army, doing similar jobs, that are rather less well known than that).