Where did you buy it? Are you 100% sure it's a UK version of the phone? Samsung make different versions for different global regions which support a specific network bands only. You may have a grey import if you bought it online from anywhere other than Samsung themselves
You may be out of luck then as the radio hardware inside the phone is the limiting factor. They can't just add support for another county via a firmware update
No, don't start messing with cross-flashing firmware not meant for your model. You risk breaking it completely even if it lets you get as far as flashing without point blank refusing to proceed. Moreover such questions aren't allowed on this forum