All UK employees are deemed to have an Employment contract, even if no agreement was ever documented or even discussed. Once signed, employment contracts can only be changed if both parties agree (ie neither side can change the contract unilaterally).
Agreement to change employment terms can be reached at the time of the change or may be given in advance in the employment contract, which may reserve the right for the employer to make certain changes.
It is always useful to consult employees over changes to their terms and, in some cases, it will be essential. Always record changes in writing; consider using a Change to employment terms letter, which should ideally be signed by both parties.