Don't confuse unlocking with debranding.
Unlocking is the process of allowing the phone to work with any network sim.
debranding is the process of changing the Phone's product ID to allow the installation of non network branded firmware.
You can Debrand a handset without removing the simlock and vice versa.
sounds like Debranding is what you want to achieve. It's simple to do and has been covered in the main 5800 thread but just be aware it will invalidate your warranty.
(Can always change product code, update firmware then change it back mind

)