A 40mph or 50mph road has been specifically judged as being suitable for going at 40 or 50 for example. A competent driver, in optimum conditions should be able to drive fairly close to it, though it's not a target.
NSL is different, it's often effectively ungraded roads, with a maximum of 60, but where you use your common sense.
On a straight, wide, A road, 60 may be perfectly safe, for other NSL roads it would be suicide. Single tracks, 90 degree bends, blinds, high hedges, sometimes all of these, and even doing 30-40mph may be dangerous.
I prefer to be cautious with speed on country roads that I don't know. I think that's far Sagar than dissolve assuming that any road with a technical 60mph limit is safe at that speed.
Both of the roads below are at NSL. I wouldn't want to guys the life expectancy of anyone navigating tiny lanes such as these at that speed.