As an ex, post war trained, nav, this is a fascinating thread.
For my threepennorth it depended on the chart/map used. It was common practice to use the Ordnance survey maps scaled in miles in the early days for map reading. However Mercator projection navigational charts were drawn up using Latitude and Longitude. This is far more convenient for navigation, i.e. Long lines are parallels and so lines drawn straight on the chart maintain a constant bearing, and, equally important, one degree of Lat is 60 nautical miles, so making measuring distances simple.
I am amazed that it took the RAF so long to change over from mph to knots, perhaps it was the perceived slight of taking orders from the Senior Service that upset their touchy sensibilities.