The log-in details for MyBT and BT WiFi are not necessarily the same. To log in to a BT WiFi hotspot you need your primary email address and password; it's the one you need to access BT Yahoo! Mail. Only a btinternet.com address will do, others such as btopenworld.com will not work. Neither are appropriate for the link you posted as that's for non-BT Broadband customers.
To get to the internet via BT WiFi as a BT BB customer, you need to connect to a hotspot. When you then try to connect to the internet you will be presented with a landing page where you have to confirm that you're a BB customer (the default) and enter your log-in details as above.
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