I concur that it would be better to know "the number of titles read on what platform" and/or "the amount of time spent on one platform." One thing that always amuses me about this particular metric, which some US companies also use, is that for some reason PC's always end up beating the other devices by a huge margin.
That said, I do think the demographic data is useful and illustrates a few differences between the US and the UK. I don't think it will make all that much difference, except in terms of which titles may end up going digital first.
I'd be very curious to find out what ebook's market share in the UK is as well. If they are in fact 18 months behind the US, it's probably still in the 1-2% range of all book sales.