When you list your item, and press the 'continue' tab, the UK option is on the next screen, just before you select 'list your item'. It is about 1/2 way down the screen on the left. You have to 'check mark' the box for 'UK' International exposure. Depending on the price of you item it will either be 10 cents or 20 cents, etc... If you item is under $10 it will cost you 10 cents...
I tried it on several of my items a couple of weeks ago, but didn't get any buyers from the UK, so I have gone back to the normal listing. Could be they just didn't like my products in the UK...

"If we had no winter, the spring would not be so pleasant: if we did not sometimes taste of adversity, prosperity would not be so welcome."