CONTAINS SOME SPOILERS
First off, I'm probably not the principal target audience for this. The novel appears to be classified as Young Adult, and I'm neither, and the central character is yet again a teenage girl. But on the other hand I'm a fan of apocalypse movies and "empty world" stories (The Day Of The Triffids, The Omega Man), so anything in which The Bomb goes off or 99% of the population are otherwise disposed of pretty swiftly is usually okay with me. However, in this case there's a struggle to find the right tone between the teenie romance and the horrors of post-nuke Britain, and I don't think they've entirely found it.
Essentially How I Live Now is a cross between The Famous Five and Threads: Daisy (Saoirse Ronan) has been reluctantly shipped off to cousins somewhere in Norfolk and is having a generally rotten time until she sees her strapping neighbour: sensitive, kind, good with animals. But then the endless games in the hazy summer countryside are brought rudely to a halt by unidentified extremists exploding a nuclear weapon in London. Rather than face evacuation, they decide to stay where they are, until the Army turn up and take them away to farms and military training camps. Daisy and her youngest cousin plan their escape back to the farmhouse, through the woods and the horrors of war....
It's difficult to reconcile the idyllic rural larking about (there is very little adult presence in the film) and hesitant relationship of the first half hour with scenes such as Daisy working through a courtyard full of corpses to see if her cousins are there. There's enough horrific imagery in the film to guarantee the 15 certificate that cuts out that YA/"tweenie" demographic (there's also a smattering of F-words, and a curious, very brief and entirely unnecessary nude dream sequence). It's also a very quick series of about-changes for her character: from her sullen, unsocial arrival when she's actually more interesting, through love-struck teen (a change apparently brought about by nothing more than being pushed into the river) to gritty survivalist prepared to kill.
Still, in spite of it never quite getting the tone right, I rather enjoyed How I Live Now: it conveys enough of the possible chaos and massive disruption to everyday English life to make it plausibly chilling, and frankly to keep a 50-year-old bloke like me interested when the drippy teen stuff kicks in. I can't remember why I didn't catch it on its cinema release, though, as it's generally the sort of thing I go for. I'd be surprised if I come back to it any time soon, but I liked it well enough.