Primary position

I have great difficulty doing any off-road riding or downhill riding (not to mention uphill, which is for reasons other than bike handling), and my reactions are pretty slow.
...Obviously, my bike handling is fine for the kind of riding I do, but I feel that it limits me from certain kinds of riding. I don't mind that much, by the way.

Oliver, off road techniques are a different skill set to the control skills you need on road (with obviously some overlap). Have you ever done a an MTB course? I too was crap off road and vastly improved my off road skills doing a CTC MTB training course with an excellent guy called Ian Warby.

Anyway, lane widths are to some extent a red herring, .

Which is why whenever describing primary position it better to focus on a rider's relationship to the traffic stream rather then suggest judgements in terms of 0.5m here or there