Controlled airspace is a different game entirely. Before you cross, you ask for a clearance either under VFR or under IFR. If you elect to cross under VFR, you must maintain VMC, because the controller may ask you to separate visually from other traffic. A VFR flight in controlled airspace that enters IMC is, in effect, an emergency.
I was always under the impression you could file a flight plan while airborne, are you saying (genuine question by the way) that you can't do it once you are in CAS? Also the scenario I was in was this in case I haven't made it clear, after receiving the 'May I remind you that your crossing is VFR' etc from the good lady I asked to go IFR if it would help. She said no need and
remain at 2,500' which is the altitude I was scud running at. So effectively I was being asked to remain at an altitude at which it was nigh on impossible not to pop a bit of clag here and there but stay VFR.....
I'll say again that Donny radar are always superb and there was certainly no question of the radio con being anything but friendly, I was just a little bemused at what I was supposed to do, in fact I think I mentioned it to Ghengis when he landed.
One thing that this thread has shown is that there are some shortcomings in the IMC syllabus.
Edit: Good show Clydeport. I've had a vac failure and it's a bit of a WTF moment.