I seem to remember, 6 years ago or so, a case of CRL based cabin crew having a barny after being fired by RYR. A judge then ruled they were subject to Belgium law because they had been employed for more than 6 months with CRL as their place of work.
Also there was a case, a year or so ago, with unions in Berlin and easyjet. I don't know the outcome of that one, but ez has UK registered a/c. Perhaps someone in know about both/either could enlighten us. Never let the facts get in the way of a good winge in the bar-room.