Employed women are entitled to 52 weeks’ statutory maternity leave. You must meet strict notice conditions, including letting your employer know you are pregnant and telling them, by the end of the 15th week before your baby is due, when you want to take your maternity leave.
Maternity & Parental Leave etc Regs 1999 , regs 4-12A

Parents are entitled to 18 weeks’ unpaid parental leave for each child, which can be taken up until the child’s 18th birthday. You cannot take more than four weeks’ parental leave for any one child in a year, unless otherwise agreed with your employer.
Maternity & Parental Leave etc Regs 1999 , regs 13-16

Advice on maternity and parental leave is available from Working Families – see Box J.1 .

Mothers can choose to end their maternity leave early (after an initial 2-week recovery period, or 4-week period if they work in a factory) and hand over their unused leave to their partner.