As I have said, it seems that the behaviour of alarms varies. Andy has said that his don't beep on mains failure provided that the alarms are being satisfactorily powered by the back-up battery (and I have a feel that mine are the same (which I'll check when I get back home). If that is the case, then there would be no 'warning' until the battery's charge nearly ran out. Furthermore, if the battery happened to be near the end of its life (not holding much charge, even when it started beeping (because of 'lost mains') that beeping might possibly not last all that long (before battery becomes flat, the the alarms totally non-operational.
In which case, I would want some sort of positive reminder or telltale, that the mains supply to the alarms had been lost.
Despite the fact that I am being accused of being 'incompetent' because I asked the question, there is clearly a dichotomy of views about this issue (here and elsewhere) - and, for what it's worth, in this thread you are the only person who has expressed a preference for a system which gives the householder a means of disabling mins supply to the alarms (without disabling something else they need - like lighting)
I did suggest a separate circuit, rather than a separate means of isolation, but for the reasons stated. The separate means was nothing to do without being able to necessarily isolate the alarm circuit, but rather to keep the alarm circuit to remain powered, whilst other circuits were isolated. I find alarms constantly bleeping, as I am working, intensely annoying - but that is by deliberate design.
I rarely had much involvement with domestic electrics, a little more on commercial, industrial - the norm there, was almost always a separate dedicated, and well-marked supply for alarms systems. Often, even a separate isolator, painted red.
Here, the stair-lift, and fire alarm system was a later addition, involving the addition of a separate two-way consumer unit. When, last year, I fitted a new, larger CU, I included those two circuits, but made a point of identify them with red labels, as essential supplies. Turn either off, and the annoying bleeping begins.