Government will send a test Emergency Alert to mobile phones across the UK at around 3PM(15:00) on Sunday 7th September 2025.
Government will send a test Emergency Alert to mobile phones across the UK at around 3PM(15:00) on Sunday 7th September 2025.

www.gov.uk
National Emergency Alert test to be held on 7th September

There have, evidently, been a few of these in the past. Neither my personal phone nor my work one has ever received one though. Nor has my SO's if it comes to it.
There have been regional tests but only one national test, and that was a part failure because the Three Network didn’t set up their config properly.