Get the unbranded RUU from XDA and flash it using a gold card - here are the files you need:
http://rapidshare.com/files/4196231...dio_12.21.60.12_26.02.00.09_M_test_148662.exe - Original test RUU
Multiupload.com - upload your files to multiple file hosting sites! - 1.32.405.6 WWE (Latest unbranded RUU)
Multiupload.com - upload your files to multiple file hosting sites! - 1.40.61.2 Orange UK (keep for warranty purposes)
I'll leave it with you to create your gold card as there are plenty of guides on how to do this online
First thing I'd do is make a gold card, put it in the phone and just try the 1.32.405.6 RUU - it may just work and debrand as any other handset would. If not, follow the guide below
1. Create your gold card and put it in the phone
2. Start the 1.32.405.6 RUU and when it gets to the first screen move the window to the bottom of your screen as otherwise it'll always be on top of anything else you do
3. Click Start>Run and type
%temp%
4. Go to the newest folder created and in there should be a ZIP file called 'Rom' - copy this to your desktop
5. Close the RUU and fire up the 1.15 Test RUU
6. Once at the first screen, move it down as before and browse back to the Temp folder
7. Go to the newest folder created and copy the ZIP file from the latest RUU into the folder, replacing the original one from 1.15
8. Go back to the RUU updater and continue through it
It should update your phone to 1.32.405.6 and you'll be debranded!! This is compiled from info on XDA and I've not tried it but it should work