Not sure why you think it is unusual, it is a specific requirement to use fire rated/resistant cable to supply fire panels in BS5839.
From a quick google, as I havent got a copy here:
'All mains power supply circuit cabling needs to match the colour of the ELV fire safety system cabling. This is normally red so that it is distinguishable from other electrical circuits.
The LV mains power supply cabling shall be segregated from the ELV fire system cabling so particular attention is needed when entering the item of fire safety system equipment in that it needs to be a separate cable entry to the ELV system cabling.
All cabling needs to be fire resistant and in buildings over 30m in height that aren’t protected by a sprinkler system, or buildings where there is a phased evacuation strategy, the fire resistance of the cable needs to be enhanced from the standard rating of 30 minutes to 120 minutes.'
Agreed the supply cable needs to be fire resistant that's been the case for years, I am not sure about the colour
All fire alarm cables should be of a single, common colour that is not used for cables of general electrical services in the building, to enable these cables to be distinguished from those of other circuits.
