The statutory notice obligations on employers are:
nothing in the first month,
one week for the first year, then
one week plus another week for every completed year of service up to a maximum of 12 weeks.
You can’t contract out of the minimum notice but you can agree to waive the notice .This is often done when the employer makes a payment in lieu of notice.
You can agree to extend the notice period. As long as the notice period in the employment contract is equal to or longer than the statutory minimum, parties are bound by the agreed period in the contract.