Just to chime in here on my experiences over the last few days:
+) ZERO - Number of time I have managed to get the giffgaff profile work on London Underground
+) 80% - Current percentage of stations that I can get Wifi Extra to work on London Underground with a 2004 SIM which allows Apple devices to directly extract the SIM mobile number.
+) 60% - Current percentage of stations that I can get Wifi Extra to work on London Underground with a 2020 SIM which does NOT allow Apple devices to directly extract the SIM mobile number.
Some stations : I can get both devices to work on Wifi Extra
Some stations : Neither device works on Wifi Extra
Some stations : Only the Old SIM works
Fewer stations : Only the Newer SIM works
New Behaviour when connecting : I have found the 2020 SIM (that masks that phone number in iOS) usually fails on first connection to wifi extra. You can Wait about 7 seconds - or manually recycle for it to connect to Wifi Extra (again - without a downloaded Profile). Alternatively - I have found toggling to Airplane mode and back is the fastest way to get it to connect (in those cases where it works).