So if you're on one of the smaller networks like Voxi, ID Mobile, Smarty, etc, you won't be able to access the free WiFi on the London Underground that is on offer to EE, Three, O2, Vodafone and Virgin Mobile customers...giffgaff.com/
There is however a perfectly legal and legitimate way to access the WiFi.
Request a GiffGaff SIM card, register it, download the GiffGaff app and sign in.
Once signed in, then in the menu of the app there is an option called WiFi Extra, open this up and select "Turn on WiFi Extra".
It has now enabled your phone to access the WiFi on the underground. If your WiFi is turned on it should automatically connect, otherwise just select the "WiFi Extra" network when in the station.
You don't need to have the SIM in your phone or any credit to use it. As long as the app is installed, you are signed in and have WiFi Extra option enabled you can access this WiFi. Available on the iPhone and Android App.
I can confirm this works as I have switched from GiffGaff to Voxi and installed the app on my new phone and logged in and works perfectly.