Is a professional needed?

[flameport]

What am I missing, why would it be a bad idea, and how would they be fed?

Two options - their own dedicated circuit, or connected to a local lighting circuit that's regularly used.

Own circuit has the disadvantage that it can be turned off and left like that. Battery backups are a thing, but they will eventually fail, leaving no protection.
Local lighting circuit is preferable as if tripped or turned off, people will notice immediately.

 

[flameport]

(duplicate)

 

[johnny2007]

A minor works certificate for the circuit extension/alteration as specified in BS7671, and a certificate for the smoke alarm installation as specified in BS 5839-6.

That's what I was looking for.
Thanks.
My boy initially said to them to do one but they insisted that after the grenfell incident they wer were doing all of this for free, no charge to the leaseholders.
I bet there will be charges in the next year accounts to the sound of 10 million.