It depends on whether they have an Audible US or Audible UK account. There don't seem to be any geographic restrictions on which you can have (for instance, you can seemingly live in Spain and have an Audible US account), but US and UK codes are only going to work for their respective Audible store. In short, the account you have - US or UK - dictates what code/site you use, not your location. And if you have an account for another country, for example Audible France, you're just out of luck. (On a side note, I believe Audible will try to automatically direct you to the store for your geographic region, but that's easy enough to deal with.)