Thing is, there are plenty (not necessarily in all areas, I admit) of 2 or 3 lane dual carriageways with 70mph limits. Motorways in all but name, so you can go on them with a driving instructor before you pass your test. A three lane, 70mph dual carriageway is no different from a motorway in terms of driving style/disclipine/’etiquette’.
I was lucky in that I was taught to drive, whereas I had friends who were taught to pass the test. One had never gone faster than 40mph or gone above 4th gear, never driven on a dual carriageway or a twisty, national speed limit country road. Plenty of them around here, but the instructor took a chance on it not coming up on the test.
But, in any case, I’m sure these people know what they’re doing. E.g.: “I don’t like driving near the edge of the road” not “I didn’t know I was meant to be on the left”. They just don’t care enough to drive properly. Like tailgating, amber gambling, speeding, not indicating etc, 99.5% of drivers know it’s wrong, but it’s quicker/easier/more convenient for them so they do it.