When your work fails, the consequence is a burglary or a fire – a "failure to perform" exposure that a generic electrician's policy doesn't cover.
Buy liability plus tools like any trade – then add the cover that makes you an alarm installer rather than an electrician: professional indemnity (or explicit efficacy wording) for failed-system claims. Disclose fire-alarm work specifically, keep your NSI/SSAIB standards documented, and check the overnight-tools conditions match how you actually work.