A contract of employment is a legal agreement between the employer and the employee. Its terms cannot lawfully be changed by the employer without agreement from the employee (either individually or through a recognised trade union).
Depending on the extent and likely impact of the proposed changes, your employer should meet with affected employees, or the union where one is recognised, and explain their case for making the proposed change. Employees should be given time to consider the proposal as well as to suggest alternative ways of achieving the same result (for example if cost-saving is the aim, different cost-saving ideas).
Where the change involves removing a benefit, an employment tribunal is more likely to think your employer has acted fairly if it has offered something in return for the change, such as financial compensation, and given enough advance notice before the change takes effect so that employees can plan ahead.
Your employer should not breach equality laws when changing contract terms. Consulting in advance can help your employer map out who is most likely to be negatively affected by the planned change, and take steps to reduce this negative effect.
An employee can decide to accept a change, and many terms of the contract are, of course, varied from time to time by mutual consent, for example a pay increase
Your employer must behave reasonably when making changes to your contract terms, and must not leave an employee unable to perform the contract – for example, requiring an employee to relocate at extremely short notice with no payment of expenses.
Where changes are made to your contract, employers must give you written notification of the change within four weeks.
An unauthorised, one-sided variation is likely to be a breach of the contract of employment, and the fact that the employer has given you notice of the change will not make it lawful. However, if you put up with the change without protesting, there is a good chance that you will be viewed as having implicitly accepted the change, losing your right to object to it.
You should consider regularly registering your opposition to the change (e.g. every month) and pursuing a claim in the employment tribunal for, say, unlawful deduction from wages. Any action would need to be taken very quickly, after taking proper legal advice.
In practice, a collective approach to this sort of dispute is often best, either via a group tribunal claim supported by your trade union, through protest and threatened industrial action, or by a combination of different approaches.
Sometimes, a change is so fundamental that it goes to the heart of the contract. A resignation in response to this kind of change could be a 'constructive dismissal'. However, resigning is clearly a very high-risk option, and usually only advisable when you already have another job to go to.