It's not illegal to have a 'phone switched on whilst driving and the guidance about the alerts clearly states,
"If you’re driving or riding when you get an alert
Do not read or respond to an emergency alert while driving or riding.
Find somewhere safe and legal to stop before reading the message. If there’s nowhere safe and legal to stop, and nobody else is in the vehicle to read the alert, you can listen to news on live radio to find out about the emergency.
It’s illegal to use a hand-held device while driving or riding."