www.gov.uk/alerts
So on 23rd April there will be a test of an emergency siren. This can be national or local and can be used to alert to inform the population of emergency situations such as severe flooding, fires or weather. It will come to ones mobile.
Maybe im being a little suspicious but i hardly think these emergency situations are relevant to most people locally unless the government are using these as a reason to implement such a system in case of severe national emergency such as war.