The US TV and radio EAS (where the federal or state government can automatically trigger the "takeover" of a station) makes sense for them due to the highly decentralised nature of broadcasting with lots of little locally owned stations with total autonomy over what they broadcast.BBC television almost launched this, interestingly, but it never made it through. Obviously not on phones but still interesting non the less.
The UK doesn't need it because even before the last 20-30 years of mergers and consolidations, things are fairly centralised. If the government had a need to address the nation then it'd be a quick phone call to the right people at the BBC, Sky and ITV/ITN.
Of course the US system doesn't remotely control TVs to turn on or anything, though special "weather radios" can listen for the signal. I believe the Japanese system for earthquake early warning also has that option.
