If a Fire Alarm System is deemed to be necessary in a building then it is a legal requirement that a maintenance plan is in place, otherwise, this is a breach of the Regulatory Reform (Fire Safety) Order 2005.
Quote from the Regulatory Reform (Fire Safety) Order 2005:
"Section 17 - Maintenance:
(1) Where necessary in order to safeguard the safety or relevant persons the Responsible Person must ensure that the premises and any facilities, equipment and devices provided in respect of the premises under this order.. are subject to a suitable system of maintenance and be maintained in an efficient state, in efficient working order and in good repair."
The word "must" in this section of the legislation means that is obligatory, failure to comply with this is an offence.