https://www.gov.uk/alerts
If the phone is switched off completely, it will remain silent.
Or at least that's what it says.
https://www.gov.uk/alerts/how-alerts-work
You will not receive alerts if your device is:
turned off or in airplane mode
connected to a 2G or 3G network
wifi only
not compatible
So it's a warning to all players in the several Congresses taking place that weekend.Emergency alerts work on:
iPhones running iOS 14.5 or later
Android phones and tablets running Android 11 or later
If you have an earlier version of Android, you may still be able to receive alerts. To check, search your device settings for ‘emergency alerts’.