As all giffgaff is done on line they need to use Official Documents to prove that you are over 18.
The Documents they have chosen are either a British Driving License Full/Provisional or a Machine Readable Passport from some 120 Countries.
Unfortunately these are the only Documents accepted to lift the Adult Bar and without them you will be unable to remove it.
Even an Agent is not authorized to remove it for you.
Only alternative is to use Wifi