Beagle
That one has been done to death, too. I know Pprune runs on a fast server nowadays; just as well
An example of case c) might be flying an N-reg on a UK PPL with the IMC Rating, and asking the CAA if the license+rating privileges work on an N-reg. I did this, and their reply was that the IMC Rating is not restricted to any particular reg, and that it is for the FAA to decide. So I wrote to the FAA and they said it's equivalent to their IR (implicitly, this is within the limits which the ANO puts on the IMCR). The FARs also enable foreign license privileges to apply. People may disagree but one can't really get much beyond asking and getting an official reply.
Case b) concerns the CAA even less.
Case a) is much more CAA territory. There just happens to be an awful lot of "common practice" stuff. Night flight in the UK, which is supposedly IFR, is actually treated as VFR. You can night depart from a Class D field in the UK and ask for a VFR departure (or just a "departure") and IME they won't bat an eyelid. I've done it in error myself. The departure you get is a VFR one (i.e. take off and bye, see you again) not an IFR departure with the extra protection that involves. You could probably file a VFR flight plan during official night and the mistake is unlikely to be spotted. And take a plain PPL with the night qual - how much IFR navigation training does he get? Practically zilch. I also never knew I was supposed to fly under some sort of IFR rules at night, and I wonder if the instructor knew it. You just fly a hopeful heading, looking for clusters of street lights and hope to start/stop the stopwatch at the right time (which of course doesn't really work at night, you'd use a GPS, but that's another story).
If there was a whole load of prosecutions in this area (and there certainly could be, if anybody cared, just in the plain UK G-reg scene) then we might have something concrete to talk about.