You've made it all sound very simple supply slave, but unfortunately as I think you probably know life just isn't that straightforward. Yes, I could refuse to teach extra lessons, causing my results to drop, causing me to fail my PM - it's all a chain of events. Employers can make life very difficult for you perfectly legally. You say that "compensation is limitless" - which is precisely why it's so difficult to prove and few people get anywhere with it. This isn't a rash decision, I've been thinking about it for months and I quite resent the implication that I am being rash about anything when I haven't done anything yet! The options are either to continue working ten hour days or to resign.