US iPhone in England. Can I use O2 without unlocking?

[Bonaa]

I'm heading to London next Friday and I don't want to pay the crazy AT&T rates while there. First, I don't want to unlock my phone. Can i use an O2 pay as you go sim card in my iPhone, or is the phone locked to only AT&T sim cards?

Thanks
-Alex

 

[thelatinist]

Your phone is locked. The only way to use a SIM from another carrier is to jailbreak and unlock.

 

[daihard]

Your phone is locked. The only way to use a SIM from another carrier is to jailbreak and unlock.

Do you have to do both? I've been told that unlocking will suffice if you simply want to use a SIM from another carrier.

 

[thelatinist]

Do you have to do both? I've been told that unlocking will suffice if you simply want to use a SIM from another carrier.

How are you going to install the soft unlock without jailbreaking?