Hi Harry
The size of the premises does not govern what part of BS 5839 is used, it is governed by the use of the building (i.e risk based). I have also never heard of a F&RS giving consent of such, it would make no sense. A conventional fire alarm system (1-2 zones) in place of an addressable fire alarm system as you mention would satisfiy the requirements of BS 5839-1 in most small offices. Regarding british standards, though they are not law, using them may help prove due diligence in a court of law (if the correct one is used).
I would imagine that any suitable and sufficient FRA would also recommend the catagory/grade of AFD to be fitted, though ultimetely what suitable and sufficient means is a matter for the courts to interperet. If the responsible person (non competaent person most likely in this scenario) decides to specify an AFD system not in line with British standards they would be in contrevention of artical 13 of SI 2005/1541 and find it hard to prove due diligence.