What follows may be helpful even though not directly relevant to courses. Yes, motorway phones are c 1 mile apart but it can vary with terrain and junctions. What doesn't vary though are the marker posts, although some may be missing mid junction. Small white post, blue type figures.
They are serial numbered from the official start of the motorway. The top number is Km, the lower one is 100m. So, example, a marker post on the M6 might have a 40 as the top number, with a 3 below. That means it 40.3 Km from the start. Next post will be 40 with 4 beneath (if you are going South it will be 40, 2). So it will go 40,4, 5 etc then 41,0, 1 etc. On a quiet stretch you can time yourself between 10 or 20 markers, no need to count just note the numbers, and calibrate your speedometer.
The markers are triangular, and the least known but potentially most useful feature is on the side away from the carriageway, an arrow pointing towards the nearest phone. You should never need to walk more than half a mile, if you made a random choice it could be 0.9 miles. Less relevant in the days of mobile phones but helpful years ago.