On the giffgaff website via this page
http://giffgaff.com/unlock/samsung-galaxy-s3-phone if you click Orange from the drop down list it says it can be unlocked for £12.99 within 20 minutes on the MobileUnlocked.com website and it has 4.5 stars. You get £3 discount by going through to the site via the giffgaff page