Buddy says:  The options an employee has when an employer seeks to impose a unilateral variation of contract on him are as follows:

• Agreeing to the variation you have imposed;
• Resign and claim constructive dismissal;
• Refuse to work under the new terms, and force the employer to take what steps it thinks appropriate; or,
• ‘Stand and sue’, i.e. working under protest and seeking damages (either for breach of contract, or for unfair dismissal).

It seems that your employee is attempting to ‘stand and sue’, however if an employee is working under protest in this way then he must abide by the new contractual terms that you have imposed, even though he is doing it under protest.  If he refuses to work under the new terms (in this case if he is refusing to work in the extended geographical area) then this is likely to be viewed as a failure to obey a reasonable instruction.  In this scenario the disciplinary procedure should be followed in relation to his refusal to work as requested.

Additional info

The options available to an employee who faces unilateral variation of contract were are as listed above, this was recently reaffirmed in the Employment Appeal Tribunal (EAT) case of Robinson v Tescom Communications.   In this case, the employee agreed to the changes 'under protest' but then refused to work under the new terms.  The employer hence invited him to a disciplinary meeting and ultimately dismissed him for failing to obey his employers reasonable instruction.  The EAT held that the decision to dismiss him was fair as, having agreed (even though under protest), the could not then renege on this agreement.  The dismissal was within the band of reasonable responses.

In such a scenario an employer should always ensure they follow the statutory disciplinary and dismissal procedures, as failure to do so could render the dismissal automatically unfair and result in an uplift of any compensation awarded.