Every, and I stress every, networkis obligated under the Mobile Operator's Association (MOA) Code of Practice to prevent access by minors to adult content. I was a member of the working party that drafted the CofP back in 2002.
It has absolutely nothing to do with OfCom save that they accepted, nem. con., the actions of the MOA.
Every operator will ask a customer to prove their majority by a variety of methods. giffgaff have selected either driving licence or passport as the most appropriate for a 100% online offering.
They do not either have to ask you or seek your agreement to grant or prevent access.
giffgaff staff will not, under any circumstances, lift the content bar without your provision of the requisite document(s).