^ OP wanted the digital display in miles so he could mean either the DIC or the HUD or both.
Whether both the DIC (cluster) module and the HUD module are sent (or fetch) a speed signal from the BCM/ECU/ABS module (or whatever module collects speed signal data) I don’t know. Whether the speed signal reading is sent or fetched and then convert it to either kms or miles within each module, again I don’t know. It could be that the module which collects the speed signal data calculates either kph or mph before it sends i5 down GLAM, so this module may need its firmware updated.
Vauxhall in the UK should know these things but likley they wouldn’t give two hoots about helping as OP didn’t buy their product. Vauxhall will not want to support OP and others that side step their products and denying them their profit by people importing their own.
What I do know is that GM codes many of the vehicle modules using your VIN number as a security measure. The features can be activated within the module only if defined in a GM database against your VIN so the GM diagnostics equipment (and thus dealers) can’t get around the limits imposed by GM. As such you can’t install features your specific car didn’t come with.
Oddly in the USofA, I believe you can purchase the codes to enable some features and this gets written into the GM database against your VIN. No such luck down under. So you need to find someone to programing or reprogramming your module via hacking the firmware. I believe
@TazzI does such hacking...
So a new or used Chevy cluster will still need to be programmed to the OPs car which if a dealer did it, it may loose some of its Chevy unique features.
I’m happy to be corrected if I’ve misunderstood the limits imposed by the GM design.