Dont think there is any legal requirement for a communal fire alarm system unless the building is over 18 or 20m high. if there is some form of smoke ventilation (BRE shaft or actuated windows) then this will need to be controlled by smoke detectors on each communal lobby.
Each flat only requires a standard domestic type smoke detector (mains operated with battery backup).
emergency lighting and signage will be required to the escape stair/route to provide minimum 1 lux.