How can I unlock my HTC One V locked by 3 (UK Carrier, www.three.co.uk)?

I have a HTC One V which is provided by 3 (www.three.co.uk) under contract. I recently bought a Nexus 4 from Google Play because of the price drop, and I'm using my 3 SIM in the Nexus which is working fine. I want to sell my old One V and know that I can do it a lot easier if the device is unlocked so it can be used on any network. After looking around online, cellunlocker.net quotes me $14.99 for an unlock code and Three themselves quotes me £15. I've heard from some friends that they were able to get their phones unlocked for free, if so, how would I go about doing this? I'd rather not fall victim to a marketing scam to get a code that I could get for free.