First it would be a mistake to install a mix of systems. Part 6 alarms (stand alone linked detectors) are for domestic use. If installing an electrical alarm in the main hall then it would be crackers and wrong not to make any required heat detectors part of this system, e.g. As part of an L5 system.
As to whether an electrical fire alarm will enhance the level of fire safety of relevant persons could be argued either way and fire risk assessment could be used to provide an outcome of a full alarm system, manual gongs or a shout of fire. Any of these outcomes could be satisfactory, the key is the fire emergency plan for the building, who uses the hall, how is it used, is anyone in charge such as a hirer, what instructions are available on how to keep the premises safe from fire and the action to take in case of fire, how the alarm is raised and what people should do if they discover a fire or hear the alarm whatever that is. If this is achieved by an electrical alarm so be it - it never ceases to amaze me how many assessors or alarm companies will default to a full L1 system, it's easy and lucrative for them but if they had ever sat on the committee responsible for fund raising maybe they would be a bit more imaginative and work it through.
Worst outcome "What's that noise?" "Not sure I think it might be the fire alarm"
Better outcome " ladies and Gents there appears to be an emergency please leave the hall immediately using the exits and assemble in the car park whilst we check the toilets and call the emergency services"
My opinion only.