Representation:

an employee who wishes to work on "under protest" while reserving the right to resign and claim constructive dismissal if the employer insists on enforcing changes to his terms of employment has to be very careful. If, even under protest, he works on under the new terms and then subsequently refuses to do so the employer will have grounds to dismiss him. That dismissal will not be unfair dismissal if, as in this case, it was for the simple reason that it was because the employee did not keep his side of the bargain.

For relevant general notes go to Changes to terms of employment