I had same problem with my iPhone 4S 5.1.1 which was also locked with 3 UK network. I tried many things , also bought a solution from http://www.superunlockiphone.com/ which never worked . I wrote to them few times for money back which I never received.
While I was searching online I found GEVEY ULTRA S SIM. I did some research and ordered it and worked for me. Now iPhone is unlocked . I hope it will work for you as well . Good luck