I'm with you...Waterstones and WH Smith are expensive (and Waterstones is very frustrating to search). There have been rumors about Borders starting an ebook store here in the UK, but I haven't heard anything recently...will go away and find out.
I sometimes use an ebook price comparison website called 'ebook price'. She has been adding more ebook stores to the list and it is worth checking if you can't get a book from your usual sources.
http://www.ebookprice.info/