You need to check the terms of the contract. If the contract says something like "15 hours, but may be requested to work on other days", then there is likely no breach of your employment contract.
If there is no such term, called a variation clause, then your employment will have potentially breached your employment contract. If they summarily dismiss you for not accepting this term, which is a breach, that would be unfair dismissal. However, you can only bring an unfair dismissal claim if you've worked for the employer for 2 years. Unless the dismissal is based on discrimination of one of the protected characteristics or it is "automatic unfair dismissal", neither of which seem to apply to you.
In short, suck it up.