Mate, don't worry about being fired. It's a bit stressful but if you're fired for time off that's covered by a doctors note then it's just a case of a trip to an employment tribunal, where they'll end up having to pay you for the work you missed as well as reinstate you or pay you off

My friend was fired for eating rotisserie chicken at Asda, and they had no proof, so they had to give him 2 months pay and put him back on the counter.
He got another year of eating free chicken out of them before he moved jobs :D Perhaps you should explain to them that getting on your case is only increasing anxiety? Also, I don't think it's legal for them to ask you in without paying you either, so if you really want to be a cunt you could make them pay you