Your employer can only change your contractual terms in the following ways:
1. By receiving your express approval to the changes;
2. By consulting with you and forcefully introducing the changes (called unilateral change of contract);
3. By giving you notice to terminate your employment and then re-engaging you on a new contract, which incorporates the changes.
If the changes are introduced forcefully then you have the following options:
1. Accept them, start working on the new terms and don't take the matter any further.
2. Start working on the new terms but making it clear to your employer (preferably in writing) that you are ‘working under protest'. This means that you do not agree to the changes but are only working them because you are forced to. In the meantime try and resolve the issue with your employer either by informal discussions or by raising a formal grievance.
3. If Option 2 does not get you anywhere then you may wish to consider taking the final step and resigning without notice. If you do so, you would need to resign without undue delay so as not to give an impression that you had accepted the changes. Then you can claim constructive unfair dismissal in an employment tribunal, subject to having at least 1 year continuous service with your employer. You would have to argue that your employer had committed a fundamental breach of contract and that you were left with no other option but to resign. Remember that you only have 3 months from the date you resign to submit your claim.
If your contract is terminated and then you are re-engaged on the new terms, then that could potentially amount to an unfair dismissal and you can make a claim for it in the employment tribunal. The same rules of 1 year's continuous service and the 3 month time limit to submit your claim would apply.
I hope this helps. Please press the ACCEPT button. You would still be free to ask any follow up questions. Thank you