From Businesslink.gov.uk:
“The minimum statutory notice period which must be given by an employee is at least one week’s notice if employed continuously for one month or more by that employer. This minimum is unaffected by longer service.”
“Right to waiver
Employers and employees can both waive their right to notice – even though they cannot contract out of the legal minimum period.”
So you only legally have to give one week’s notice whatever it says in your contract of employment. Whether your boss is allowed to increase it as and when they see fit, I do not know but as you only have to give a week legally I wouldn’t worry about it.