I thought Apple was only renting space to developers [slashdot.org], and got a fixed percentage from them. Isn't setting/raising prices something developer's should decide to do? Or are things somehow different in the UK?
Basically, when you sell something in any of Apple's stores, you choose a price tier in your default currency, and prices in other currencies are based on that price combined with the current exchange rates. For example, if I create a book right now, and specify tier 10 everywhere, that's $9.99 in USD, or $13.99 in CAD. If the Canadian dollar increases relative to the dollar, in a year, tier 10 could be $9.99 in the U.S. and $12.99 in CAD. In theory, the amount paid will always be approximately equal to $9.99 in USD.
To add further complexity, Apple provides some alternate price tiers that let you charge lower prices in developing countries, and for books, even lets you set per-country price tiers, IIRC, which could distort pricing even further... but that's a side discussion. :-)