You may also find that Fictionwise will have a different range of titles from Mobipocket's EU store, at least in English Language works. This is because the right to publish books in the English language is traditionally sold in two territorially defined tranches: North American rights and UK and rest-of-world rights. This made sense back in the age of sail -- when nobody wanted to ship pallets of books across the Atlantic -- and it's still written into most publishing contracts. As ebook rights are subsidiary rights (the publisher buys the right to publish in a given territory; this right includes a bunch of subsidiary rights such as "record audiobooks, publish in hardback or paperback, publish electronically ...") you therefore find different ebook editions from different publishers in different territories.
I should like to note also that Mobipocket might be breaking the law by charging VAT at 19% on ebooks sold to UK residents, as the VAT on books in the UK is rated at zero percent; although it's possible HMRC have imposed VAT on ebooks at a different rate. (If so, it wouldn't be the first time they've done something that dumb.)
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