Install Nemesis Service Suite (NSS) and Nokia PC Suite. Connect your phone and ensure its connected to your PC via Nokia PC Suite mode (will show up on there). Close Nokia PC Suite. Fire up NSS.
In the top right corner, click "Scan device" and it should detect the phone.
Click the "Phone Info" tab then "Read".
In the box, it should show some phone info but most importantly, the Product Code. Change the Product Code from whatever it is to "0568238" for UK & Ireland (Non Branded) or "0543713" which is Euro1. Use any you want, but en-gb pwns and is it both
Tick the box next to the Product Code and click write. You should get a confirmation message.
Make a backup and run Nokia Software updater and let it find the latest unbranded firmware for you