It's all on sammobile.com

Go to "firmwares" then "firmwares and files" and use the drop down filters to select the gt-i9300 and united kingdom - BTU (which is the unbranded CSC).

XXELL4 is the one you're after.

 

or wanamlite

 

some people seem to be saying the XXELL4 is o2 but it has no branding as far as i can see. Except it has the joyn app which appears to be a vodafone only app so that confused me. But in answer to your question download the XXELL4, it is unbranded

 

XXELL4 is a Samsung build. Where:

XX = Region (Most European Countries)
E = Still not sure
L = 2012
L = December
4 = Revision 4

Both UK Unbranded (BTU) and O2 are based on this Samsung build. You cannot identify the carrier by the build alone. Both are different iterations of LL4

 

I gave you step by step to get it. You have to use the filters as I described

 

*sigh*

Can't have followed my instructions then

Its dead easy.

1. Open a browser and type www.sammobile.com in the address bar and press enter

2. When the page loads, at the top is a row of purple buttons, the third of which is "firmware". Press this...

3. This loads a second row of buttons underneath the top row. The fourth button is "firmwares and files". Press this....

4. Scroll down to the bottom of the page where it says "latest firmwares". Just above here are the filters you need to set. Set them as follows:

Type:smartphone
OS:android
Model:GT-i9300
Country:United Kingdom - BTU

...and press send

Happy Christmas