Ive just downgraded my OTB 1.1.2 iphone to 1.1.1. I know its not possible to unlock yet as we have a different bootloader to the phones in the US but ive read in a couple of places you can use a o2 pay&go simcard?

Ive tried this and still get no service? Do i need to install some kind of unlocking app?? Im not really sure and dont want to brick it.

Ive tried searching but you cant search o2 as its less than 3 characters long so apologies if this has been asked before and its probably quite a newbie question.