I take it that the author of the plan has not had the opportunity to be posted as a SO3 or G3 with the British Army.
The plan is way under resourced. having one position in 5 manned leaves a gap of 800 meters between positions, which is hard to cover even with a 7.62mm machine gun, and there will only be at most 3 that could be brought to bear at any one point. Meanwhile, the attacker gets to choose the place, time, and strength in which to attack, easily taking out 1 or 2 positions, and then has easy access to the secure zone to place explosives on the supply route, and with good information, even attack a supply convoy. There is even a good chance of making an escape, while the reaction force in en route to the scene of the action.
The estimate of 48,000 guards for the security zone is therefore, too optomistic. Additionally, some of the guards will be unfit for duty for whatever reason, and the guard force has to be fed, supplied, housed, and transported to and from their positions. And security has to be maintained for all of those activities.
In the end you end up with a force of well over 100,000 to maintain a supply route for a force of just over 100,000 troops. Fantastically expensive to do that.