As already stated, Red is the preferred Colour, and concealed cabling is acceptable, as long as the cable is appropriately supported when concealed in false ceilings.
I can only suggest that you approach certified Fire Alarm Companies and STILL check out any concerns on this forum. Yesterday I looked at an installation by a reputable FA company, third party certificated, and they had installed mains powered domestic smoke alarms, installed in White FP, very nice lookng installation, linked via a relay and Zone monitor (Correctly Zoned, texted and addressed)into an addressable Fire alarm, because there was not enough capacity to add 8 new devices.
No Certificate left on site, but the original quotation confirmed it would be to BS5839 Part 1 Category L3
We maintain the FA system for the Landlord, there are 30 spare addresses on the open protocol loop, it would probably have been easier and have cost less, to do the job correctly, each device would have had text, and the new devices would be monitored for faults, shared the back up power supply with the Fire Alarm System, wouldnt need batteries changing every couple of years.....
The Mind boggles!