For anyone in the UK on 23 April there is to be a test of a new National Emergency Alert System at 3 pm.
It seems that almost any mobile device, of any nationality, will be contacted.
Even if it is somehow just UK devices you will certainly be aware of it happening, from everyone's reactions.
More details here-
https://www.gov.uk/alerts?fbclid=IwAR275jZseTvohg-gfPDKmyX1ec8gDxVnrsa82anx6IIsowjRUbENCjBYY_4
EDIT (6 April)- Time now added- this time now falls between the London Marathon (that morning) and the FA Cup Semi-Final (at 4.30pm), and also avoids the times of Church Services in the morning.
EDIT (20 April)- If travelling by public transport, do not take any immediate action, but follow staff instructions and announcements (this is TfL advice)