After having a discussion with several mobile telecommunication companies; I have been told that if you have had a contract (on that specific phone) and it has been paid off, or have been on Pay as You go (with them; in that phone, and same number) for at least 9 months...
... Then you can call them up and they will give you the Unlock Code for your phone "as a good will gesture, for using their network".
I have used this technique successfully with Vodafone and Three (Vodafone was PAYG... And Three was an "old" contract I paid off).
If they don't feel like co-operating, then you can ask to speak to their supervisor or just visit his lovely page on the GiffGaff website; which could also help you out with unlocking your phone 🙂
Michael James Swan