Question for UK readers: Where do you buy mobi/epub?

I have a UK reader who is wondering about e-book availability for the Guardian series (obviously not Demon Angel, but that’s coming!) I’m trying to help her out, but I’m never certain when I’m looking at bookstores whether the ebook is unavailable, full stop, or whether it just appears unavailable to me because of my geographical location (which is also why I can never tell whether a Kindle edition is available in Canada, sigh).

So as a general question: If you are a UK reader, where is the best place to find mobi/epub books?

And more specifically: Has anyone been able to buy Guardian books through them?

Any help is much appreciated!