Does a period of maternity leave break continuity of service?

No. The Employment Rights Act 1996 and the Maternity and Parental Leave etc Regulations 1999 (SI 1999/3312) make it clear that periods of maternity leave do not break continuity of service.

Absence on both ordinary and additional maternity leave must be taken into account for the purpose of calculating statutory benefits, such as redundancy pay, that are based on length of service. Ordinary and additional maternity leave also counts as service for contractual rights based on length of service.