From the ACAS website
Time off for antenatal care
Do pregnant women get time off to attend antenatal classes?
All pregnant employees are entitled to time off with pay to keep appointments for antenatal care made on the advice of a registered medical practitioner, midwife or health visitor. Antenatal care may include relaxation classes and parent-craft classes. Except for the first appointment, the employee must show the employer, if requested, a certificate from a registered medical practitioner, midwife or health visitor, confirming the pregnancy together with an appointment card or some other document showing that an appointment has been made.