For employees on a permanent contract for a specific amount of hours per week, your employer cannot reduce your hours or change the terms of contract without your consent.
More details can be found on https://www.gov.uk/your-employment-contract-how-it-can-be-changed. If your contract was changed without your consent then you can raise a complaint. For further details visit https://www.gov.uk/your-employment-contract-how-it-can-be-changed/dealing-with-problems
However, for employees working on a zero contract, there is not much you can do and your employer can change your hours as and when it suits. More details can be found here https://www.gov.uk/contract-types-and-employer-responsibilities/zero-hour-contracts