You don't have many details so it's a bit hard to say what was off course. The British AC have an OVR value of four FP (two FP MGs tripled and halved to three plus two for AFV; five FP if you use the IIFT). They immob a truck on a final DR of five. The British have lots of armor leaders, of course, but despite what you seem to say the vehicular dust and light dust DRM do apply on an OVR. Did you read the EXC in light dust for "OBA" as "OVR"? Also the minus one DRM for OVR in Open Ground only applies to Infantry, not to trucks.
On the first turn a British AC might be able to just barely reach the track and OVR one vehicle. During setup there is no vehicular dust put behind the Italian vehicles. I assume the British are paying double MP costs to enter accessible to hammada, although that does not figure in to the first turn that much.
In the Italian first turn the Italians dive into vehicle dust (v-d) as much as possible as a matter of course. With some careful planning the Italian trucks might form a "dust Luffberry circle," with each truck obscured in the v-d of the truck ahead of it. The truck carrying the 65mm gun can end up in the v-d of at least one and possibly more vehicles. The crew unloads in the first turn as a Passenger (Italian MAON C). In the second Italian MPh the Italian crew unportees the gun off the truck, becoming TI. They are in at least one v-d, which at least cancels the hazardous movement, plus light dust. The L3s might be set up to cover this somewhat delicate operation by discouraging the British from parking adjacent to the hex with the portee truck. Since as you note the Italians tend to become befuddled it is a good idea to station the L3s in hexes that would be suitable if they fail their TCs, perhaps one on each side of the gun's anticipated hex. If you set the gun up in a hex like 30u5 with an L3 on either side I don't think it can be OVR on the first turn unless the british are risking hammada immobilization. On the British third turn the 65* is ready to fire.
I believe the scenario tends to favor the Italians as is recorded in ROAR. As it is a vehicle vs. vehicle scenario, a few lucky DRs might tip the balance suddenly in one direction or another, but in most cases it will average out, especially given the generally weak weaponry.
JR