Note: BS 5446: Part 1 covers smoke alarms based on ionization chamber smoke detectors and optical (photo-electric) smoke detectors.

The different types of detector respond differently to smouldering and fast flaming fires. Either type of detector is generally suitable.

However, the choice of detector type should, if possible, take into account the type of fire that might be expected and the need to avoid false alarms.
Optical detectors tend to be less affected by low levels of ‘invisible’ smoke that often cause false alarms.

BS 5839: Part 6 suggests that, in general,

optical smoke alarms should be installed in circulation spaces such as hallways and landings, and

ionization chamber based smoke alarms may be the more appropriate type in rooms, such as the living room or dining room where a fast burning fire may present a greater danger to occupants than a smouldering fire.