Email from Amazon
Hello Karl,
This is Janelle with Alexa Customer Support. I understand you would like to know whether your Echo, which you have purchased from the U.S can be used to link to your Prime account in U.K. or not. Let me assist you with this.
Once the Echo becomes available in the UK, which is on Wednesday, September 28, 2016, you will be able to migrate your Echo over and have it functioning properly.