Not only is AFD not required in the common parts- the guidance does not require the provision of any fire alarm in the common parts. So there is nothng to connect the heat detector to.
The degree of compartmentation, limited travel distances, ventilation to staircases and corridors, fire resisting self closing doors to each flat, are considered sufficient safeguards even if a serious fire occurs in an unoccupied unit. Sometimes it does go pearshaped though and there are a number of news stories on google where the fire brigades have had to evacuate large parts of a purpose built block of flats.
But to balance that there are many more examples of buildings converted into flats in which the compartmentation was not in place, that have been much more serious and a risk to both occupants and firefighters, alarms or not. See the recent fire near the Albert Hall in london as an example.
Unlike most situations nowadays, the safety design and evacuation strategy of flats still relies on the brigade attending and intervening- even to the extent of opening the vents in the staircase where these are manual.
The responsible persons in all other purpose groups are not allowed to take intervention by the brigade into account within their emergency plans.
