It's updated on an annual basis to reflect government statisticians' best guesses as to what best reflects the goods consumers are buying. The annual rejig also shows what British consumers are eating and drinking as well as what they aren't.
According to the ONS, ebooks “represent a significant and growing market,” the statistics office said. Current best estimates suggest that ebooks accounted for 13% of the UK book market in terms of volume and around 7% in value terms.
Curiously enough, ereaders are not part of the basket (though tablets and smartphones are included). This makes me wonder what exactly the ONS thinks those ebooks are being read on. As of last April somewhere around a market survey firm reported that a third of Brits owned an ereader. As of November tablet penetration was estimated to be around 16% in the UK. If these numbers are anywhere near accurate then ereaders may actually be more common in the UK that tablets.
It's a pity the ONS didn't notice.