I hope this helps anyone who has difficulty connecting their iPhone to London Underground Wifi. I recently updated to iOS9.3 beta (but was unaware that this was actually the cause of the problem).
The issue I experienced was that whenever I connected to ‘VirginMedia Wifi’ the connection would drop straight off. No login screen, nothing.
I tried the usual things like hard reset, ‘forgetting’ the network, deleting cookies in Safari and finally resetting the network. I did NOT want to wipe my iPhone.
The fix was this – navigate to ‘Settings’ > ‘Cellular’ then scroll to the bottom of the list and toggle Wifi Assist off.
I also have ‘Settings’ – ‘Privacy’ – ‘Location Services’ – ‘(Scroll to bottom) ‘System Services’ – ‘Wi-Fi Networking’ turned OFF.
Hope this helps. Took me forever to fix.