Is your phone actually unlocked (or at least locked to O2)? Even an unactivated giffgaff SIM will display signal bars/dots. No signal bars is a pretty clear sign that the phone is still locked to another network.
Have you tried / can you please try the SIM in another (unlocked) phone? If it works then the phone is still locked.
I donate 100% of my SIM activation payback to charity, please help me boost the donation