your right to ask this question!
A lot of people think because it is not notifiable work, that it can be done by anyone, which unfortunately this is true.
But it's also true that most electrical work within the home can be done by non qualified persons.
But then the problems start the reports and form filling which need a knowledged of inspection and testing and the use of calabrated testing devices and the expected result.
The work your brother has done, does not come under part p notification but does need an MWC(minor works certs) as it does not extend to the provisions of a new circuit but is an addition to an existing one.
With your brother installing fire alarms he may have some of the test equipment needed to do the required tests and fill in the report. but does he have the required knowleged to do this?
Test would be
Earth Fault Loop Impedance
If your selling the house, might be worth having a PIR(periodic inspection report) done, some estate agent and potential buyers insist on this.
This will assure your installation is healthy.
Some electricians won't sign off work they have not performed and within the requirements of BS7671, they state that an electrician should not, but some will do this, if they are willing to take the responsability of that circuit on.
Like I've said before your brother can do this if he has the kit and knowledge, after all his made the addition so is responsible.
I know that may not help.
Most electrician will do PIRs though.
Ask around and find out.
Work's hard to come by at the moment you may get lucky
Rolly, if your brother is an electrician who installs alarms as well, he can copy a minor works cert from regs book and fill it in for you, if he only installs alarms who fits the mains power to panel for him? maybe he can do it for you,
It is also possible to buy a pack of certs (10) which are NCR copied so there is a duplicate for record purposes.
On the other hand it may be if attempting to sell, the potential purchasers solicitor/agent will request a full 'periodic Inspection Report' of whole house so this would be included, so no need to do it twice!!