You lock it to a particular network not unlock it. If you unlock a phone any sim should work bar 3 I think. If you do not want to follow 'Fruit bat's' suggestion there is bound to be somewhere near you that will do it,for a price of course, just look in your local phone shops.
I just recently had a Samsung phone unlocked at a local shop. It was previously on contract with Orange. I looked at sites on the Internet and found they would send me an unlock code with instructions for £25. My first thought was what if it doesn't work and the price seemed a bit steep too. My local shop charged me £10 and I was then SIM free to put any card in.
My advice would be to find a local shop - takes about 20 minutes.