Now to go back on topic, my question wasn't about the type of detector but are the views about detectors in roof voids.
If your question is simply "should I or should I not install some form of fire detector in my loft?" then obvious the answer would be yes, it's a sensible idea. The more detectors the better, as long as they aren't prone to nuisance alarms.
We have some detectors at work that rely on thermal imaging to detect the heat signature of a flame, I expect they would work rather nicely in a loft! Mind you, I think they cost a few thousand pounds a piece.