No they cannot.
The employer cannot unilaterally vary your contract where a fundamental term of the contract such as pay is concerned.
You can take out a grievance against your employer using the company's grievance procedure. Every company must have one by law. Alternatively you can threaten to take them to an Employment tribunal for unlawful deduction of wages which you can do whilst still employed by them.
You can also threaten to leave and claim constructive dismissal on the basis that they have breached your contract by changing a fundamental term without your consent.
Whether a constructive dismissal claim succeeds would depend on the amount of difference in pay that this change has created.
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