GPJ, like jeltex and MVP, is an outdoor jacket. Put it on if going outside and it is a bit cold.
Some ppl do seem to just mooch around in them indoors because they can’t be bothered putting on a tie to wear a jersey and I can almost understand that - stupid rule about the tie/jersey.
It’s what our parent force do mostly these days. Most of the time I’ve been with the regulars they are either in MTP, ignoring the rules or just wearing a GPJ permanently.
Those some people include most regulars, again purely because the DI Cadre pushed the dress regulation change through that means if you are at work in short sleeved shirt and cold you need to change into a lomg sleeve shirt and tie to put your jumper on!
No, you could put a tie on the short sleeved shirt.
By a strict reading of AP1358 they can’t. (Unless the foolishness has been amended).
You are correct!
Although DIs cant see through jumpers 
I know at least one station where they were checking.
I stand corrected! I was sure that used to be a thing
Noooooooo.
Dont take the magic away!
I thought you all had x Ray vision. Immediately gifted to you at the same time you all get a pace stick welded to your hand! 
Haha it’s all to do with the secret power sharing welcome to club hand shake at the end of the course
I maintain you should be able to wear the long sleeve with an open collar… Such a ridiculous rule.
Regulars can do exactly as we can do - put a jersey on over a short sleeved shirt if you want, but you must add a tie.
I too miss the old days of being able to wear a jersey without a tie - though, I wear a jersey so very infrequently that it doesn’t affect me often.
The rules say you can’t do that and their are seriously places where the WO’s check
AP1358 is not updated particularly regularly.
I’d be hugely surprised given the recent “cooperation” if the dress and clothing policy committee gave us authority to write the option into our regulations if they weren’t happy to do it themselves.
I have never understood the aversion to ties. If your shirt is the right size they make no difference to the fit around your neck.
I always wear my jumper when it’s cold, because mooching around in a short sleeve shirt and GPJ looks, IMO, gopping.
I have an aversion to jerseys and a dislike of having my collar fastened.
Ties are an antiquated and outdated fashion accessory which should be consigned to history.
I actually think it’s a shame that men’s civilian attire isn’t bringing back more from the past.
Style these days is a joke.
I’d say that there’s very much a time and place for a smart tie.
The problem in uniform is that to get an RAF shirt which fits comfortably in the collar usually means that it fits like a tent everywhere else.
My short sleeved shirts are 37 because they fit best in the chest. In the old days I could just throw a jersey on top if I had to and keep the collar open. Now I’d have to squeeze my 38 neck into a 37 buttoned collar and tie.
I wear my short sleeved shirts year-round. The GPJ is an outer garment and shouldn’t be worn routinely indoors (for no better reason that it looks dumb, aside from regulation) and I find that the solution is a simple one… I turn on the heating when I arrive in winter.
That’s nice if your building has heating that doesn’t predate Julius Ceasar.
However, I just put up with the cold and wear a short sleeve shirt.
Ok, clearly I am going to annoy many but…
Our buildings heating is at best… useless. As I find the jumpers irritate my skin, I do wear the gpj indoors. BUT, with longsleeve shirt and tie.
Much more comfortable and still smart.
In summer, it’s obviously no jacket and either longsleeve with tie or short sleeve no tie.