Okay, I've had another look on VCDS. The following applies to my 2010 facelift FR, but I assume is the same for all FL cars.
I've tried all of the country codes and none of them turn the in-line KPH display to MPH, which is consistent with what I thought previously.
I have worked out
how to turn on the larger MPH display in the MFD though, which is as follows:
1. Recode the instruments controller to Rest of World via VCDS.
2. In the settings on the MFD you'll see a new option to show the alternative speed display (the in-line KPH display). Turn this off.
3. This will reveal a new option in the MFD Data menu for the digital speed display. Turn this on and it will be added to the scrollable screens on the MFD.