harryE - if you do want epubs, BooksonBoard has a UK site which, sometimes, offers a book cheaper than any of the other UK retailers I have come across. Also, ebooks.com, whilst a USA site, occasionally can sell a title to a UK resident and its prices can be good.
Overall, I find WH Smiths is my most frequent epub site but, since the opening of the Kindle UK operation, I'm with HarryT on what is the most sensible option to take (providing you have access to a pc).
Oh, and I have never had a problem with Calibre on my Mac. In fact, I prefer using my Mac and Calibre than my pc and Calibre.