I was about to suggest Waterstones, but I see you've already found that. One bookstore with a reasonably large selection of ebooks from major publishers is
Kobo Books. Not that they're necessarily much better than the other big names when it comes to geo-restrictions, but at least they won't show you books only to deny you a purchase once you've signed up and checking out.
Otherwise, to get the books you want from most commercial bookstores, you'll have to use US/UK billing or shipping addresses and perhaps a proxy too.