I work as an assistant manager pub company they have

Many thanks for your patience. There are a few ways in which an employer may try and make changes to an employee’s contract of employment. These are by:
• Receiving the employee’s express consent to the changes.
• Forcefully introducing the changes (called 'unilateral change of contract').
• Giving the employee notice to terminate their current contract and then offer them immediate re-engagement under a new contract that contains the new terms.
If the changes are introduced without the employee's consent, then the following options are available:
1. Start working on the new terms but making it clear in writing that you are working ‘under protest’. This means that you do not agree with the changes but feel forced to do so. In the meantime you should try and resolve the issue either by informal discussions or by raising a formal grievance.
2. If the changes fundamentally impact the contract, for example changes to pay, duties, place of work, etc., you may wish to consider resigning and claiming constructive dismissal. The resignation must be done without unreasonable delay so as not to give the impression that the changes had been accepted. The claim must be submitted in an employment tribunal within 3 months of resigning and is subject to you having at least 2 years' continuous service. You would then seek compensation for loss of earnings resulting from the employer's actions.
3. If the employment is terminated and the employer offers re-engagement on the new terms that could potentially amount to unfair dismissal. However, the employer can try and justify the dismissal and the changes if they had a sound business reason for doing so. This could be pressing business needs requiring drastic changes for the company to survive. If no such reason exists, you can make a claim for unfair dismissal in an employment tribunal. The same time limit of 3 months to claim and the requirement to have 2 years' continuous would apply.
Finally, it is also worth mentioning that sometimes employment contracts may try to give the employer a general right to make changes to an employee’s contract. As such clauses give the employer the unreserved to change any term, so as to evade the general rule that changes must be mutually agreed, courts will rarely enforce such clauses. Nothing but the clearest language will be sufficient to create such a right and the situation must warrant it. Any attempt to rely on such clauses will still be subject to the requirement of the employer to act reasonably and can be challenged as above. This is especially true if, as in your case, the changes are only allowed on a temporary basis, such as to provide cover.
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