Out of interest, what's stopping me, as a resident of the UK, from actually buying this?

Nothing happens. You'll get to a screen where they thank you for your interest, but apologize about not being able to provide the book to you.

Thom_Merrilin_UK, on , said:

 

What a shock, only the US and Canada. Do Tor really think there are no other countries?

 

This is eBook...regions don't matter.

This is an eBook, that's why regions matter. Importing physical books is a lot easier than eBooks. It's mentioned in the faq on eBooks here and if you try to purchase eBooks without using an American billing adress you're unable to check out.