If your employer has recently started not complying with the terms of your contract, you should object, perhaps by raising a grievance. You should seek the help of your trade union, which will have expertise in this area. You can also seek the help of a mediator. Ultimately you may need to complain to a tribunal.
If, because of the behaviour of your employer, you no longer feel able to continue working, you can resign your 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/ go 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'.
In any event you should obtain early advice before taking such a drastic step as resigning - ask a union rep or legal advisor for information about your exact situation.
