I don't think you were Able to do it, but these days it's very easy to switch, it's in the account settings I think. You are asked if you want to switch to a .com account and click the button and away you go!
Hmmm, I'm not sure how to do that. I know that you can't order from the .com kindle library from the UK but I'm not sure if that's linked to your IP address, your email or where your kindle is at the time?
I am registered on the co.uk store but somehow my prime is registered to the .com store. If I change country to the USA then will I be able to join the kindle lending library thanks to my Prime account?
Also apart from that, I believe the USA store is cheaper and has a wider range of books...?!