Sorry mate, but giffgaff have a very simplistic take on this problem. The giffgaff adult content bar can only be removed by providing details of a UK passport or a full/provisional UK driving licence ( NI driving licences excepted due to format difficulties ).
Credit card age verification is underway though ( coming soon apparently... ), but not in place yet.
See here for general guidance, and advice about difficulties in filling out the form. http://community.giffgaff.com/t5/Help-Ask-the-community-got-stuck/How-do-I-lift-my-Adult-Content-Restriction/ta-p/3510729
Of course, you could always ask a teenager how to get round it. They always know.