I agree with you that this is a problem -- not so much on price, but on availability -- or, rather, the sheer difficulty of finding what you want.
The Waterstones site is terrible; it's almost impossible to browse ebooks in a specific genre, or to do multiple ebook-specific searches. Other retailers and publishers offer ebooks from their sites, but they suffer from the same difficulties with searching and browsing as Wasterstones (Penguin Books is a good example).
In short, there's nothing remotely as convenient as the Amazon Kindle store (which is the main reason I bought a Kindle, although I admit that's not an easy thing to do in the EU).
Like Harry T, I'm not so bothered about the price issue, although I'd be happy not to have to pay so much.