Apologies if everyone knew this already, but when I visited the Books on Board website today I saw that they have a uk bookstore now, which I have never noticed before, and haven't seen mentioned here either.
Books aren't necessarily cheaper, but they do seem to have a good range, and are offering their rewards as they do on the US site. I checked a couple of books I've been browsing recently, and one was more expensive than waterstones, the other one cheaper, and both were available as epub.
There have been a lot of discussions recently about buying books given geographical restrictions, and this might help.