The Fire Safety Order does affect vacant premises, one of the reasons landlords/owners are included as potential Responsible Persons with the definition
"?relevant persons? means?
(a)any person (including the responsible person) who is or may be lawfully on the premises; and
(b)any person in the immediate vicinity of the premises who is at risk from a fire on the premises,"
meaning that an empty building could still be a risk to those adjacent/nearby.
The required level of precautions and maintenance may be less and most emphasis is usually on security, removal of combustibles and isolation of power so there is little opportunity for fire (including arson) and little opportunity to spread, with alarms, e-lighting, extinguishers, etc often being left out/powered down/removed. Sprinklers are often left active, although in a vacant premises a wet pipe system could be prone to freezing and rupture so these are sometimes isolated.
Insurers are the main drivers - there may be little life risk, but there remains property risk and they may require greater precautions to be kept in place.