In my experience, (which includes highland cattle!) - you really do need 2 rows of barbed wire no more than 9ins apart, so the cattle actually can't get their heads between.
We have stock netting topped by 2 rows of barbed around a field next to the highway. The council tried to tell me it was a hazard if a cyclist fell against it. I pointed out it would be more of a hazard if the cattle got out and caused a traffic accident. Further more, as there's a deep pond in the field would they take responsibility if a child climbed over the wooden fence that they wanted, and drowned in the pond? I heard no more and the barbed wire remains.