giffgaff are required by OFCOM to restrict the access to sites with adult themes through its data network. If you are 18 or over you can lift the restriction with either a passport or a driving licence.
The passport is no longer restricted to the UK, but needs to be in "machine-readable" format. This means at the bottom of your passport photo page there are two lines with many chevrons (“<“). Each one of them has 44 characters in total.
Full instructions here:
There is NO OTHER WAY to lift the adult content restriction at this time, and giffgaff are bound by OFCOM in this regard. Sorry.