Amazon UK imports a large stock of American books, and you can order from them directly and very cheaply. That's how I got ELANTRICS and the MISTBORN TRILOGY several years ago.

The value of having a UK-specific version is that they get into a lot more high street bookshops, however, and it financially benefits the American author more thanks to the royalty arrangments using the UK-US exchange rate.

In addition, British paperbacks are frankly far better-made than American ones (although American hardcovers are usually superior and much more hard-wearing), so it's quite nice to have a book where the ink doesn't run or pages start falling out at random intervals.