Well, we don't have flash floods or dust storms or tornadoes in the UK, so a ational warning system is irrelevant.
The argument for "local" alerts, that is to say an alert sent from a single mast to the immediate surrounding area, warning of some disaster is equally hard to believe. I can't imagine any scenario where a person living in a small Welsh valley would need an alert sent to their phones to warn them about the weather. They could see it from their front door.
A more plausible explanation however is to test the technology required to control the mass movement of people.
If an alert can successfully be sent to say a local area, then it also means that other information could also be sent to that area. When all new cars are "connected" via their on-board Sim, and they are/will be electric, it will take nothing more than the press of a button to Geo-fence an entire area so no vehicles or mobiles can operate within it. Remember, this alert will disable your handset and you won't be able to use it until you read the emergency message. They can do exactly the same to your vehicle.
I can easily imagine a scenario where your private vehicle will only work if the government allows it to be switched on, or where whole areas can be electronically isolated so you can't drive there. So no more congestion charges or LTN's, they will just switch off your car via a text message to stop you driving there.
It also means they can see how many handsets have received the message, which gives an over view of population density and movement.
It's all very sinister, we have willingly walked ourselves into a totalitarian nightmare.