If you are interested, the government does periodic risk assessments of potential incidents that could affect the country. The current document is
here.
For lovers of risks assessments, there is a table on page 9 which charts the likelihood and severity of potential events. He highest severity events are a pandemic disease which wouldn’t probably require immediate action and a major CBRN event which definitely would. For the highest likelihood, you have a terrorist attack in a public place which might be helpful to know about ASAP.
Text messages and tv announcements are great but don’t have the ability to localise to the level of the emergency alert system to one mobile phone cell. Imagine a major CBRN event, you could tell people in the zone to hide indoors but people outside of the zone to stay away. Never let humans make judgements about risk if they don’t have all the facts known at that time., we are terrible at it!