The traffic volume on most such roads is quite low and most people are not driving very fast so serious accidents are relatively rare.
I don't have any statistics handy but my impression is that accident rates are higher on roads that have higher volumes and more complex junctions. I too would be interested to see some statistics on accident rates for the different kinds of road in the UK.
As @
senghenydd says many such roads are hundreds, in some cases thousands, of years old. Widening them to allow for two lanes along their entire length would be hugely expensive because many of them are a metre or two below the level of the surrounding fields or they run through narrow valleys. And of course you would be reducing the size and hence productivity of the fields that you take land from. It's certainly debatable whether the cost would ever be paid back in reduced cost to society through reduced accident rates.
My experience of driving in Devon and Cornwall is that as soon as a road is wide enough for two cars the average speed rises dramatically and I suspect that this would have an unwanted and equally dramatic effect on the fatal and near fatal accident rate. By wide enough I mean wide enough for two ordinary cars to pass with a few centimetres to spare between then and to the edge of the road; remember that because many are sunken below the field level the edge of the road might well be a two metre high solid wall of mud and rocks with overhanging bushes and trees.
