Martin Webb
Hello Martin,
From your message it seems like you are not able to access your websites from Brazil but they are accessible from UK. To access your website from Brazil, it should convert its name into IP, which is then used to gain access to the resource you have requested. As different ISPs are involved in this request routing, this IP could be restricted in any point. However, I didn't find any recent report related to Google IPs restriction in Brazil. I can also access the mentioned site from the Compute Engine VM in us-central1 region. You can use 'tracert' on Linux machine or ‘traceroute’ on Windows machine to find the hops the request is travelling through. Using this command will show you in which point it breaks the connection. Bellow is the command format:
# tracert [ hostname ]
You can check the issue with using MTR tools as well to see packet-loss status of each routing path. Additionally, you can try using your appspot.com link (http://[YOUR_PROJECT_ID].appspot.com) directly to access your website rather using a custom domain name.
As your website is accessible from different places except Brazil, it seems like the issue is related more to networking rather than to App Engine itself.
Hope it helps you to investigate the issue further.