This is simply not true. I live in the UK, don't have a UK passport or driver's licence,use GiffGaff and was able to remove the filter by typing in my Polish passport number just fine!
"Thanks for contacting me about removing your adult bar. Don't worry I'll look into this for you now.
Unfortunately due to a change in process recently we are only able to remove the adult content bar with a full driving license number or with a valid UK passport."