O2's latest priority offering is a free adult cinema (odeon) ticket valid Monday to Thursday. Says you can claim one per week.
If you don't know what o2 priority is have a look here:
You can get the moments with giffgaff as it'll accept the number as an o2 one. Not sure about the other o2 Virtual networks.
Try downloading the app for you android/iphone if you have no joy via the website with giffgaff/tesco mobile