Need help jailbreaking 4.2.1 3G o2 UK iPhone.

[HappyLibrary]

Hi,

I won a 3G in a competition over a year ago. Bit slow, bit laggy, had to have it replaced twice due to overheating/inability to make calls, but aside from that, it's good. It was most recently replaced around September/October.

As I won the phone, I used my old sim card, which didn't let me browse the internet unless through WiFi. As you can imagine, this made my phone pretty useless. Now, however, I can afford the contract to allow me to browse.

I just got a new contract today (O2) and the guy in the O2 shop said I should really update my firmware. I haven't bothered for months as I found it just made my phone horribly laggy each time. So I updated today to 4.2.1 I believe, and it's slowish again. And I can't even change my home screen background, as the guy in the shop told me I could. I posted on another forum about this and they told me to jailbreak it and have given me guides.

Now, they all seem to be for AT&T/T-Mobile phones, and have all this stuff about unlocking them, and I just don't want to take a risk yet as I'm with a UK carrier, O2, and I don't know how this would affect it. Some videos I found had 4.2.1 in the title, then said in the video that their file only worked with 4.0, and then said about it being tethered, untethered, base bands etc. I don't know what is most recent and what's out of date. The whole thing is horribly confusing.

So, can anyone please, very kindly, recommend the best jailbreaking program for a phone with the following stats? I knew jailbreaking existed but today is the first time I've ever researched it.

Carrier: 02 9.0
Model: MB4898
Version: 4.2.1 (8C148)
I'm using Windows 7.

Also, there's no way that jailbreaking can cause my phone to stream data without my knowledge, right? Since once I go over the 500MB usage, I'll get charged £3 per additional megabyte. Stupid question, I just want to be sure. Also, before I even consider this, I'd need to see a tutorial I can refer to in the event I ever need to restore the phone back to it's original condition. I've already had to get it replaced twice, so in the event it needs replaced a third time, I need to make it look like it hasn't been jailbroken.

EDIT: I have read "CompleteJailbreakable", and the YES/NO columns mention about the jailbreak only being unlockable if you use a certain iPad baseband. I don't want to risk doing this unless I know this will not affect my o2 network, I don't even know if my handset is locked to o2 or unlocked or will become locked if I jailbreak it. I also don't want to touch the baseband, it seems quite advanced and it looks like messing around with it will definitely throw my warranty out the window. It also says there is no SHSH to restore, and I don't want to do this if there's no way of restoring my phone back to how it original was before jailbreaking, I'm guessing that's what that means. Installing custom firmware myself also looks like a bad idea.

Just letting you know I have read around, but I still don't want to take any risks without someone clarifying what I'm doing.

Thanks.

[iYeow]

Since your iphone is the official carrier of 02, you don't need to unlock with Ultrasn0w and you don't need to do the ipad baseband.
From what i see, you have restored to stock 4.2.1, now you can just jailbreak it with redsn0w 0.9.6b6 and only check install Cydia on your 3g iphone.

[HappyLibrary]

Since your iphone is the official carrier of 02, you don't need to unlock with Ultrasn0w and you don't need to do the ipad baseband.
From what i see, you have restored to stock 4.2.1, now you can just jailbreak it with redsn0w 0.9.6b6 and only check install Cydia on your 3g iphone.

Alrighty tyvm, my baseband is 05.15.04, does that make a difference to whether or no I can use redsn0w 0.9.6b6?

Yeah I just updated it to 4.2.1 through iTunes.