@ unstableair - great thinking, but I tried too and got the same result.
I will raise with Amazon Support too.
Update: Got a very speedy reply from Amazon Echo Suppport:
"Thank you so much for your feedback to add ZIP codes outside the US to the Echo. This sounds like a fantastic suggestion! I too would like to see this added. Hopefully they implement this sooner than later.
I have
specifically passed this suggestion along to our development team for the Amazon Echo, as your comments and suggestions will help us improve the Amazon Echo and offer better service to our customers."
Fingers crossed it will be added