I have searched and am guessing (though it doesn't appear to be explicitly said anywhere) that my Speed display will switch automatically from MPH to KPH as I ride off the ferry into France. However, I wanted to be sure as there doesn't seem to be a switch within the App (iOS) to select it.
I wanted to be sure before we head off on a journey and I'm fumbling about trying to set it if it doesn't do it automatically.
The switch from MPH to KPH has to be done manually when are reach another country by going to Settings -> Regional &Voices-> Units.
I have no idea how I didn't see that option in settings. Blindingly obvious now ;-)