British BS 9251 & NFPA 13D & 13R Obstructions

From a sprinkler contractor in the UK - We have a British Standard BS 9251 which is based (loosely) on your NFPA 13 D/R for the use in houses, flats and care homes and so on.  One area which is very grey to us and not included in the design standard is the distance from obstructions such a pendant lampshade which lower than the ceiling or a smoke detector or emergency light fitting.  

I know most contractors are using 300mm (1 ft) as a minimum from a pendant drop light fitting but then you ask them why no one can tell you.  for other obstruction such as a smoke detector many any using 4 times the depth as a method.

For beams and larger continuous obstructions, it's much easier as most just use a bean rule in EN BS 12845 (our equivalent NFPA 13).

I would be very interested in understanding the approach which you take for such things and very much looking forward to your assistance.

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