@BevanCR so you have two ways to think about the problem.
On the broadband side, if set up properly a battery backup system will keep your router and ONT if you have one powered up during a power cut. Either the exchange or the green cabinet down the road also has battery backup so your internet will keep going for as long as the batteries last. Plugging your phone into the router if you have digital voice will mean you can make calls, or use you mobile for WiFi calling if you have a suitable device.
Mobile phone masts also have battery backup, so that should also keep a signal to enable you to make calls.
So between two ways you can make your 999 calls for a number of hours after a local power outage.
