Wiring smoke /heat alarm system

Assuming that you have applied and paid for part P building regulation consent to carry out DIY electrical work, then 1.0mm 3 core and earth is needed for basic alarms, although for a DIY install it would be more sensible to use wireless radio interlinked alarms as no connecting cable needs to be run between the detectors. A 6amp BSEN60898 type B circuit breaker would normally be used. Remember that the LABC building control officer will need to call round to check your work and sign it off as compliant with building regulations and BS7671:2008, so depending on the fee charged by your local authority it may be cheaper to hire a registered electrician to do the job.

I agree with previous answer however I would like to add that the preferred method of installing heat and smoke alarms is not to provide a separate circuit, but rather to connect it to the lighting circuit. The reason for this is that when the batteries run low and the alarms start to beep, if supplied by a separate breaker, some people would switch it off until they can replace the battery which sometimes never happens. If supplied from the lighting circuit, residents are less likely to just switch them off as they would also loose their lighting too.