Is it acceptable to have copper pipe with brazed joints for a domestic sprinkler system rather than the orange plastic pipes?

Copper tube has always been acceptable for sprinkler systems. The reason it is not commonly used is probably the higher cost of copper installation. This is a partial extract from BS 9251:2021 Paragraph Commentary 5.13 Pipes and fittings: Copper tube conforming to BS EN 1057:2006+A1, R250 should be used above ground. Capillary fittings should be joined by soldering or brazing with alloys with a melting point of not less than 230 °C and conform to BS EN ISO 9453.