The United Kingdom has now tested its first-ever country-wide alert service on millions of phones on Sunday 23rd of April.
Residents of the UK confirmed that the service went off at exactly 15:00 UK time as their phones emit a loud alarm and vibrated with the message:
This is a test of Emergency Alerts, a new UK government service that will warn you if there’s a life-threatening emergency nearby
The message also included instructions to follow in such a case in order to keep everyone safe. See the screenshot below: