UK fire safety law just requires a management system (& where necessary, appropriate equipment) to be in place for raising the alarm. Some guidance (including that given by central Government) says that it must be an electrical fire alarm. That is nonsense. You are right that it is perfectly acceptable where the risk is low, and where the premises are small and simple, for manual bells, gongs, whistles and the shout of 'fire' to be acceptable. Whether it is acceptable in the case to which your refer is simply not possible via an internet forum so you will need to justify the control measure in your FRA findings. Doors will obviously dampen noise travel, so if you have a door or two between the ground and first floor, this could be a problem. As would a cellular office or inner room, or where staff need to wear headphones etc. You need to think of others in and around the building. Do they need warning? Plus any system must be resilient and available when required. For example, whistles do tend to get stolen (and who wants to blow a whistle that's been God knows where and not cleaned for months??) and I would never advise the use of aerosol fed air horns as there's no way of knowing if the supply levels in the can are adequate But as a general concept, for small low risk premises, non electrical systems of warning can be deemed suitable & sufficient Good luck