I have an imported echo that I use in the UK. Once the UK software was available I Deregistered it from my account US account and set it up to use my amazon.co.uk account. I was able to set the location correctly but Prime and audible still didn't work. Got the following email from amazon yesterday, gave them a call, apparently the fix needs to be done by their engineers so they've raised a case for me and it should be fixed in a couple of days. Thought I'd mention it here incase anyone hasn't been emailed.
Dear customer,
Amazon recently announced that Amazon Alexa devices are now available to customers in the UK. You currently have an Alexa device that was registered to your Amazon account prior to Alexa being launched in the UK. Because of this, you may not have access to features like Amazon UK Prime Music, skills that developers have created for UK customers, or shop through voice on Amazon.co.uk (coming soon).
If you'd like to update your Alexa devices, please contact us so we can help you with this process.