I’m not surprised.
Based on my experience, most employers don’t actually know what a contract is, nor are they aware and therefore not concerned about workload factors. And further the intricacies of grounds for breach of contract is ‘nobody’s’ business, from what I have seen of employers’ attitudes. [Hence why so many silly cases end up at Employment Tribunals etc.]
I regularly refuse to take on excessive work - and to do other dodgy stuff. I can do that because I know the GMC’s 31 Standards for doctors inside out (i.e. I have the means to rapidly - within seconds - find relevant clauses, not from the internet). With that knowledge I inform them which standards I would be breaching in taking on excessive work (or other dodgy practices). Caution: I am not saying that GMC standards define contractual terms.
What I am saying is this: “Force me to breach any GMC standard and you render the contract for my services void. Explanation of a consequence is not a threat - it’s informing ‘you’ what you are doing.” - and I tell them that in pretty much those plain terms.
Fortunately perhaps, I’ve only had to ‘walk’ (suddently) after an explanation, once in 30-odd years of medical practice. In other situatons - no explanation - just notice and quick departure.
Obviously, I am not everybody - nor is everybody me. I do things differently. But I do recognise that I’m meant to do what ‘everybody’ else does.