Personally, I'm not too sure why somebody would want to wear their wangs on their cabbage kit but I guess if CAS is doing it, that's all the top cover you need. There are times and places where you don't want to be drawing attention to the fact that you are driver, airframe. Need I say more?
Also the PM is fine with it as well.
The PM doesn't know the first thing about military dress regs and probably couldn't care less if CAS had appeared in a pink feather boa. Remember that on a recent occassion when the PM addressed the troops in the sandpit, his turnout was a bag of sh1te, looking like a double glazing rep having a lunchtime sharpener in the slug and lettuce. Great guy.