Probably something to do with the fact that all RAF drivers are destined for fast jets until they fall off that escalator and get on another. Oxygen masks make the cockpit not the place to have a dose of anti-pollen behaviour at 420kts.
That is not the case in the FAA or AAC - neither, unfortunately, and mind-boggingly, have fast jets anymore - and you are likely to join to fly rotary.
G
