Amazon have added in-app store option into the Windows Store app enabling browsing and purchasing of books from the app. Unfortunately at the moment it only works for users with Amazon.com accounts so if you have a UK account when you are taken to the Amazon website, hopefully they can expand it to other markets soon.
You can get the app from the Windows Store
This release includes an in-app store for touch optimized devices along with improvements to the reading and library experiences.
(In-app store is available in English for customers with an Amazon.com account)