I have recently moved to O2 and have been trying to find out how to connect to WiFi Extra. After talking to customer support and searching without much success, I stumbled across a way.
So hoping that this helps someone in same position as me and is easier to find.
Your phone should automatically connect to WiFi extra (at tube stations) if it cannot get a mobile signal. I could see WiFi Extra but not connect. If you have same, then do the following and you should find you connect automatically to WiFi Extra.
1. Open WiFi settings
2. Click Add network
3. Set network name/SSID: Wifi Extra
4. Set security: WPA/WPA2/WPA3-ENTERPRISE
5. Choose EAP method: Sim
6. Click Save
