You don't need to install the UK software to buy books from Waterstones. I have never installed the UK software and can buy ebooks with no problems. I have the US software installed so I can access the free books at the US ebook store.
I don't think you can have both versions installed on the same computer. But someone may come along who knows more.
You can also buy ebooks from WH Smith. I also like Books on Board and Fictionwise.