This is a great guide on how to flash via ODIN:
forum.xda-developers.com/showthread.php?t=1671969
To pick up the firmware go to here:
forum.xda-developers.com/showthread.php?t=1646610
This is the latest firmware for the UK
I9300XXBLG8 Download (Thanks To SamMobile!)
Base Firmware: I9300XXBLG8 (4.0.4)
Region Europe
Country: United Kingdom
Carrier: Unbranded
Build Date: 26 July 2012
Modem: XXLG8
CSC: OXABLG8
Change List: 928452
This shouldn't wipe but I could be wrong. You may also need to change your CSC but doing this WILL wipe your phone.
EDIT: I'm on Three myself and I flashed an unbranded version almost straight away. Easy enough to do