Once a parent has given notice of their plans to end maternity or adoption leave and either parent has informed their employer of their intention to take SPL, the notice given to end maternity or adoption leave and the notice given to activate SPL are binding and cannot be withdrawn. There are only specific times that plans to take SPL can be withdrawn (eg so that a parent can change their mind about ending maternity or adoption leave), which are:
when within 8 weeks of the parent giving notice to end their maternity or adoption leave, it’s discovered that  neither parent qualifies for SPL or statutory shared parental pay
when notice was given before birth and the intention to withdraw the plans is expressed within the 6 weeks following the birth, or
when the partner of the parent taking maternity or adoption leave dies
For more information, read Shared parental leave.