I would think your dealer can install a new face plate for your speedometer gauge? Have you checked with them?
UK cars must have both MPH and KM for speed and in the I don't think European cars have the MPH as standard..... I have certainly never heard of it.
My e28 (1987 535is was purchased in the US and has both the MPH and KM it can be changed by a push of a button). In Canada we only have KM faceplates on our speedometer gauge, whereas in the US they have both KM & MPH
I never understood why BMW does not make this a standard for all cars they build with dual readout KM & MPH or MPH & KM depending on what country they ship to.
Anyway, I recommend you contact the dealership. Do let us know how you make out I like to change mine also to have both displays KM & MPH. I do travel to the US quite often and speed readouts are posted there in MPH ... I don't think the US will ever go Metric
as will our English cousins
Good Luck