In answer to the question in bold:
Just because a road has the national speed limit does not mean you can travel at 60mph down it. It is a guide and that only. Roads are sign posted as national speed limit as on some parts of it you can go up to 60MPH but on many bends etc. you can not! It would be stupid to have signs at ever bend with a new speed limit and therefore we are given the freedom to use our common sense and travel at a speed that we see fit.
The highway code states that:
The speed limit is the absolute maximum and does not mean it is safe to drive at that speed irrespective of conditions. Driving at speeds too fast for the road and traffic conditions is dangerous. You should always reduce your speed when
- the road layout or condition presents hazards, such as bends
- sharing the road with pedestrians, cyclists and horse riders, particularly children, and motorcyclists
- weather conditions make it safer to do so
- driving at night as it is more difficult to see other road user
It also states:
Take extra care on country roads and reduce your speed at approaches to bends, which can be sharper than they appear, and at junctions and turnings, which may be partially hidden. Be prepared for pedestrians, horse riders, cyclists, slow-moving farm vehicles or mud on the road surface. Make sure you can stop within the distance you can see to be clear. You should also reduce your speed where country roads enter villages.
The statement in bold covers corners on country lanes as many you cannot see around and therefore need to reduce your speed accordingly.