AFIAK there isn't. It's strictly a matter for agreement as part of the Contract of Employment. If an employee has an issue such as you mention, the employer should be approached about it in the first instance. If the employee decides to leave within the agreed period and so break the contract, the employer is free to take an action (unlikely according to the conventional wisdom) but the Court might decide that the notice-period was unreasonably long and thus unfair, I suppose. Like a lot of this stuff, a Court has to decide the rights and the wrongs at the end of the day.