Answer:
The distance on maps is measured by the little scale that is located in the bottom right corner of the map in this case. I can not measure it precisely over a picture like this but I would say that between London and Ashby on this map there is 100 miles of distance, and between London and Exeter there is around 150 miles of distance. This is measured by using a ruler and applying the distance that is on the scale to the distance in inches on the ruler.