As giffgaff is all done on line they to verify Age by means of official Documents and have chosen either a UK Driving Licence Full or Provisional or a Machine Readable Passport from some 120 countries.
Without either of these Documents you will be unable to lift the Adult Bar.
Even an Agent is not authorised to lift it for you.
It has been suggested that giffgaff allow Credit Cards be accepted for Age Verification and I believe that giffgaff are considering this.. They should give it priority to help members like the OP who do not have the requested ddocuments.