They need to be installed correctly by a competent person. Not necessarily an electrician,
If competent then he/she is an electrician.
I know in Wales the fire service will install smoke alarms, yet I had a problem with mothers house, the social services installed linked to a call centre smoke alarms, removing the existing ones installed by the fire service, so when I got house re-wired I did not include smoke alarms as already fitted, on her death social services came and removed the equipment, so house sold with no working smoke alarms as council removed or disabled them.
Seem to remember Nest doing a linked smoke alarm, which seems odd when their wall thermostat is one of very few at that price not linked to TRV heads.
However as to UKCA marked equipment not so sure, as yet not seen the logo on a single bit of equipment. I would think Scotland may have a supply problem.
As to mains powered, I noted in my house only smoke alarms in the flat under the main house, and there is a RCBO labelled smoke alarms so assumed mains powered, however they started beeping, and had to renew the PP3 battery to stop them beeping, so really not sure if mains powered.
Also linked, likely was part of the intruder alarm, which from state of wiring I would assume does not work.
I remember working for a firm and being called to a noise in the security hut which they did not know what it was, it was the panic button in main reception which had been in error luckily, to summon help in an emergency.
Alarms are only any good if people know what it is sounding for, like the reversing bleeper, unless a well known sound, useless, having it say "beware I am reversing" is only any good for English speakers, and so many items produce white noise also useless.
I have never heard my smoke alarm sound, at work yes, guy goes around with a smoke generator and tests them all, but at home that is unlikely to happen.