You can easily change your country settings for your Amazon account on amazon.de to a UK address, if you so wish, but it's an all or nothing approach: you can't just have K4PC connect to the UK store and, for instance, your Android Kindle app to the DE store, unless you have them on different Amazon accounts.
Go to:
Mein Konto > Mein Kindle > Laendereinstellungen
It will show you what store you are currently connected to and let you change it to any other store.
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