Most of the advice on 'Places of Public Assembly' is aimed at buildings which are likely to have members of the public in who do not know the building, such as nightclubs, bars and the like. These buildings will be divided into numerous rooms and storage areas and so do require detectors and other precautions to protect escape routes.
But churches are, in general, a large single volume open building in which any fire would be be seen quickly by the occupants and would be unlikely to hamper their escape. In addition the occupants are likely to be familiar with the building and its exits.
In my opinion it is not necessary for a church to have either a manual fire alarm system or one to L2 or any other designation under BS 5839 unless the FRA indicates that there are circumstances which require it, for example regular public concerts or many funerals and weddings when there may be many visitors present or where a particular hazard exists.
The IFE Heritage Special Interest Group spotted the problems in the advice about Assembly Buildings, particularly when applied to listed church buildings, and has been working on better guidance for churches since its formation three years ago; it is hoped we will be publishing the advice in the not too distant future!