One thing to keep in mind about the UK waterstones ebooks store.
The few waterstones employees I have spoken to have said that the prices will be close to UK Paperback book prices in most cases, and higher for new releases. So £6 to £8 pounds for most and higher for some.
I don't know about you, but buying from US based stores still looks like the best bet, with prices like this. They Might be wrong, and prices might be cheaper, but living in and with Rip off Britain, I'm not keeping my hopes up.