What can I do if some of the terms of my contract of employment have been invalidated by my employer's actions?

If, because of the behaviour of your employer, you no longer feel able to continue working, you can resign employment and claim 'constructive dismissal'. To succeed you would have to show that your employer's actions had breached the contract and that such a breach was so serious as to be fundamental, going to the heart of the contract.

Not all constructive dismissals are unfair. For example the employer might successfully argue that it was necessary to make changes to the contract and defend a claim for unfair constructive dismissal on the basis of 'some other substantial reason'.

Constructive dismissal cases are generally considered to be hard to win. You should therefore take early advice before taking such a drastic step as resigning.