@G2EWS
Have you considered looking at the road from a slightly different point of view?
You keep mentioning '20mph' and '40mph', these numbers are meaningless in terms of safe driving*. If the road is safe one day at 20mph the next day, with a bit of dampness on the road, it might be only safe at half that speed, or less.
The safe speed is determined by the experienced judgement of the person who is determining the speed of the vehicle they are in control of, and that varies depending on all the circumstances of the moment.
(I have slowed to a complete stop to then crawl around some bends in roads because as I started to drive around it I was faced with blinding low sun. Stopping for a moment means that I don't drive into something/someone I can no longer see. Slowing early for it also means that the following drivers don't suddenly run into the back of me because they were not paying attention. It also means that they don't drive into the wreckage of my car were I to have crashed around the bend.)
Also consider that many of these roads were in place at a time when the fastest thing to travel along it was a horse at walking speed where the rider was able to see above the height of the hedgerows on either side.
There was no reason for the roads to be safe at the speeds expected by motorists, regardless of the posted speed limits.
Also, whether the road had signs warning of a sharp bend or not, it is the responsibility of the driver to 'read' the road ahead to determine the appropriate course of action.
If, for example, you have a straight ribbon of tarmac ahead of you and then there appears to be greenery right across that tarmac in the distance then it stands to reason that there is no more 'straight ahead' road for some reason. Could be a bend, left or right? Could be a dip or the brow of a hill? Could be a tunnel? It would then be prudent to slow down until the situation can be assessed further. That process continues up to the point of possibly stopping and wondering why there really is greenery right across the road. Maybe because it is a fallen tree due to a storm? Hitting 3 tons of fallen Oak at 20mph will hurt, hence stopping before getting there. Hopefully it will just be a bend in the road that you can continue to drive around at a safe speed.
This still isn't 'having a go at you'. There is an awful lot of learning that happens after one gets a license to drive (or gets on the road in any capacity or vehicle type). It is worth appreciating that new understanding whenever, and however, it happens.
After 30 years of driving (road, race, cars, motorbikes, trucks, LGV, PSV, and bicycles, and also teaching driving to others), I am still learning new things to take into account, and modifying my driving and thought processes to suit.
*Safe drivers don't need speed limits to tell them how fast they can drive. A safe driver will (almost) always be driving a road at considerably less then the currently posted limits to allow for the changing conditions and hazards.