TL;DR: Remove any custom nameservers you have set on your machine and you should be able to connect fine
Earlier this week, I spent a good 15 minutes or so trying to get online at a Starbucks, but the Openzone connection page kept timing out. I initially thought it was a problem with the store, but when others could connect fine I had a think what it could be.
As it turns out, I had set up my Mac to use Google's DNS servers (220.127.116.11 and 18.104.22.168) as I was testing something at home. Without the BT Openzone's DNS servers, I couldn't find the login page to connect to the internet. Removing any custom nameservers and using the ones that the router supplied fixed all my issues and I managed to connect instantly.