I completely agree that the limits are just that, limits rather than targets but the accident in the video would have happened at slower speed, just the consequences (which are only hinted at, not shown) may have been less severe at the lower speed.
Completely agree also about the narrow, winding lanes that are technically 60MPH limits - on many, it's all but impossible to get up to the limit, even on a bike! Did most of my formative driving down the lanes and have never had a problem apart from once when some tw@ in a 4x4 came belting round a corner. I managed to stop and get right into the bank and he almost managed to stop, just sliding gently into me. Not enough to cause any damage to me beyond a (possibly already there) slight scratch on my bumper but it did take out his headlight cluster, bumper and crumpled a wing slightly. I had no crumple zone to distort - the joys of LR 90s!