Interesting-I've an Irish phone and number so it must have come straight from the transmitters rather than to a list of numbers
It's based on the call tower that your handset is connected to.
That allows for geographically localised warnings - e.g. floods, toxic fires which require windows to be closed, escaped dangerous criminals nearby, terrorist threat, wildfires, etc, etc. - as the message needs to be received by handsets within physical proximity, rather than base on which network it is registered on (including non-UK roaming ones).
Given the number of alerts that most phones give their users every day (even, occasionally, somebody telephoning them!), and the possibility that the information contained within the emergency broadcast might just save your life, I find it very hard to understand the mentality of those that refuse to receive them.
