That's a bit dodgy innit? If your lighting is set up as one circuit to
cover
the whole house, it may be OK but if the downstairs lights are on one fuse
and the upstairs lights a different fuse, someone could pull the fuse for
the upstairs and expect the smoke alarm to be safe to work on when it's
not.

They shouldn't expect any such thing, as to connect them up in that manner
would be against the regulations, which requires all smoke alarms to be
interconnected and on the same circuit (either a regularly used lighting
circuit, or a dedicated alarm circuit).

If they are expecting smoke alarms to be distributed across multiple alarm
circuits, then they aren't competent to work on an electrical installation.
They are also not competent to work on electrical installations if they
think that flicking an SP MCB is enough to consider a circuit to be
deenergised, let alone isolated.

Christian.