As far as I know, we in the Uk do not have such things as Amber(or any other colour) alerts. I strongly suspect nobody would take a blind bit of notice of one if they saw it ! I for one would suspect the police or whoever of making a situation sound worse than it really is for their own convenience, and there's enough of that already.
You are quite right, we don’t. Childline offer a Text one if you sign up.
I also agree that most would ignore it or turn it off but if one person in a group gets it they’ll no doubt mention it and with that the word will spread just as it does on Facebook or Twitter.
I think as long as there are strict guidelines for when they can or can’t use it then it would be effective.
You should also note that there are two systems which are widely used:
AMBER (which is an acronym as well as being named after a child): which is used for missing or abducted children.
Wireless Emergency Alert (wea) which is everything else from weather alerts to national security events.
In my opinion the UK would need a totally separate one which works with how we do things and as I said, would need to be strictly controlled or targeted.