"Another thing I noticed (seeing as we are on to roads) is that on main roads, the junction numbers (which must have been changed recently I think) coincide more or less with mileage. This means there isn't a 6, 7, 8, 9 etc. You might have 3, 9, 11, then a 45a and 45b. Am I right"

Its so logical when you think about it isn't it, planning a trip is much easier if you can see that you join the I4 at say, junction 58 and where you are going is at junction 78 , you know there is approx 20 miles to the turn off so can judge it more easily.