Harry,
I do not know about the UK, but in Germany any contractual clause that is against the law is void. Usually contract even have a "severability clause" to avoid cancelling the whole contract if they messed up and demanded something nonlegal...
It is up to the person to decide whether they are morally obligated to follow a contract they signed, or not (if it is non legal, that is).
cheers,
kaas
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