Cell broadcast[1] is a standard GSM feature that civilised countries implemented as a means to send emergency messages years ago, and the UK eschewed. They're now playing catch-up.
Good or bad depends on what they use it for. Technically, it has the advantage of being a broadcast transmission, so avoiding the network congestion that an equivalent SMS or voice call to all users would cause. It's also cell-specific, which is potentially very useful with small cells. (You could, for example, send an evacuation message to everyone in a stadium or railway station. But not to the point where it'll become a useful replacement for sodding fire pagers, because modern life is rubbish.)
Potentially useful for severe flooding, fires or terrorist incidents or whatever. Less so if they decide to fill it with the sort of guff[2] you get on council or emergency services twitter feeds. Or worse, as a propaganda outlet for the Conservative party.
As for hacking, sure. Either you set up a rogue cell[3] and send a legitimate message to the phones currently associated with it, compromise a cellular operator's equipment, or you somehow find your way in to whatever government IT project is responsible for conveying the legitimate messages to the cellular operators. It's absolutely the sort of thing that 1337 h4xx0rs or disgruntled employees might do for the lols, but I'd expect they'd send something harmless like a rickroll or a "$telco suxx0rz" rather than a 4-minute warning. Hopefully they won't set the password to the sort of thing an idiot would have on his luggage.
[1] Back in the Nokia days, Orange would transmit the cell's STD code via Cell Broadcast, so you could display it on the home screen.
[2] Cell Broadcast supports multiple channels, so if they did decide to do this, they could do so in a way that you could filter out.
[3] The sort of thing the police are allowed to do for the usual policey reasons, but Ofcom will throw the book at you for if caught.
