If the employment agreement contains a variation clause, the employer may be able to make reasonable changes if there is a permitted reason for doing so. Keep in mind that this variation clause needs to be mutually agreed with the employer and employee. An example of when a variation clause may be used is where the employer may reduce your pay if an unforeseen event occurs and the business is struggling financially. Variation terms need to comply with Unfair Contract Terms Act and be reasonable, if not the employee has the right to take the case to an employment tribunal.
In the absence of a variation clause, a contract of employment is a legally binding contract between both employer and employee and cannot be changed without the consent of both parties. Any changes made without your consent will not be binding and the terms of the mutually agreed employment agreement will apply.
This depends on your employment agreement. If there is a variation clause an employer may be able to make the change and notify you within a reasonable amount of time. If there is not a variation clause, the employer must make any amendments with your consent.