My Galaxy S3 was locked to 3-UK when I got it. So I used a code I bought on the internet to unlock it. When buying the code the form asked which network I wanted to unlock the handset for. I said O2-UK as that's the SIM I want to use. No other SIM cards work on the handset except 3 and O2. If I root my handset will O2-UK still work on it? Thanks to anyone that can help.
Rooting the phone won't change SIM-lock settings, they're saved in a separate part of the phone, please see these previous questions for more info on that: Where Are The Sim-Lock Settings Stored on an android phone? and Does Unlocking bootloader affect network lock?
You can find more explaining these different terminologies here: What is the difference between: Rooting, Jailbreak, ROM, Mod, etc.?