The HM Government guide for sleeping accomodation does offer some advice when you look at the notes section at the bottom of the table on page 55. i.e. if the the flats/appartment blocks are purpose built and comply with building regs or not.
Note 1:
In the case of single self contained chalet type accommodation a Grade D LD2 or 3 system may be appropriate.
Note 2:
An individual private dwelling in sheltered accommodation should have a Grade C LD 2 or 3 system but this need not be connected to the
system that covers the common areas. Where the occupants are mobility impaired to a degree that they would be at high risk in the event
of fire; or they suffer from a disability (e.g. speech impairment) that would preclude communication with the fire and rescue service, provision
of facilities for the automatic transmission of the fire alarm signal to the fire and rescue service should be considered.
Note 3:
An individual private dwelling should have a Grade D LD2 or 3 system but this need not be interconnected with the system that covers
the common areas of the building.
Note 4:
Flats and maisonettes constructed to current building regulations, will not require automatic fire detection in common areas. However,
self-contained smoke alarms will normally be fitted within each accommodation unit.
Note 5:
The outcome of your risk assessment will determine the necessary standard of AFD required, e.g. in some cases, a BS5839-116
Category L1 system may be appropriate.