Er, I looked into all of this during a slight remodel a year or two ago. Long story short, couldn't justify the cost of any smart smokes.
Instead however (and I think this is a hell of a lot better for a number of reasons), I did get a sparky to install some standard physically interconnected AC smokes (fire angel, from memory). One goes off, they all go off. Bonus points for these guys is that you can happily purchase a mount (just need one, because obviously they're all connected) for a couple of quid which includes a dry contact relay.
Which... wooo...! Connects to one of my most favourite devices in the world, a nodemcu flashed with konnected firmware. I must have about 20 of these dotted around the house, they're awesome. Used to use the same setup connected to a fibaro ubs on smartthings before i learned to hate, well, both smarttthings and fibaro =p
With the relay, you can happily connect to non-smart devices with ease (smokes go off, siren goes batshit!), and with the addition of the nodemcu, that makes all sorts of cool smart stuff possible too (mine texts me if the smokes get set off/tested).
Replacement smokes are less than 20 quid when they invariably need to be replaced (5 years, maybe, off the top of my head?). Can't go wrong.
=)
