There is one other retrofit option that is just plug-and-play. Kidde makes an acoustic sensor that can listen for the specific alarm pattern which is mandated by law in the US for smoke detectors. That device isn’t a smoke detector itself, it’s just an acoustic sensor. It hears that specific sound pattern and sends a Wi-Fi notification. It’s sold at Home Depot for $99, occasionally on sale for $79.
Although the acoustic sensor doesn’t integrate directly with SmartThings, because it can send multiple texts, you can set it up to trigger smartthings events through the IFTTT channel at the same time that it notifies you.
It’s a pretty straightforward option that was designed to address the issue where people had existing nonnetworked hardwired smoke detectors, but they wanted to get notifications if the alarms went off when no one was home.
Works with your existing smoke or carbon monoxide alarms - designed to work with all UL-approved smoke and carbon monoxide alarms manufactured after 1999.
I have no idea if smoke alarms in the UK use the same acoustic pattern or if there are any similar devices there. But for the US, insurance companies and consumer reports seem to like this one. Consumer reviews are generally good except for people who put it on a Wi-Fi extension network. That’s not an uncommon issue for a device that has specific timing like this one.