To access Wi-Fi at London Underground stations please select Three Wi-Fi in the Wi-Fi networks and you should then be able to access the internet.
On an iPhone:
Go to Settings
WiFi
Turn WiFi
Select Three_WiFi
On Android Devices:
Open your device's Settings app.
Network and Internet
Internet
Select Three_WiFi
If requested:
Set ‘access method’ to EAP
Set ‘EAP method’ to SIM