> The country roads one has always dumbfounded me though - why some of those have national speed limits I will never know.
Why not? Even roads with lower speed limits you're required to drive at a speed appropriate for the road, the conditions, and your vehicle; the speed limit merely sets an upper-bound, and it's not really relevant whether it's achievable. Just look at the Isle of Man where there is no national speed limits: most roads outside of towns have no speed limit, regardless of whether they're a narrow single-lane road or one of the largest roads on the island.
If you set a limit, some people will drive it regardless. Even if you're supposed to drive to the road and conditions, there are enough utter morons out there who'll take a blind narrow corner at 60.
Why not? Even roads with lower speed limits you're required to drive at a speed appropriate for the road, the conditions, and your vehicle; the speed limit merely sets an upper-bound, and it's not really relevant whether it's achievable. Just look at the Isle of Man where there is no national speed limits: most roads outside of towns have no speed limit, regardless of whether they're a narrow single-lane road or one of the largest roads on the island.