Compromise agreements
If an individual reaches an agreement with their employer "in full and final settlement of any claims" this only covers claims for breach of contract and not any statutory claims. Employees can only contract out of statutory claims where there is a compromise agreement, that is a written, binding agreement executed by independent advisors such as lawyers, union officials and advice workers giving their services without payment.
(Sutherland v Network Appliance [2001] IRLR 12)