I don't have a problem with 50mph limits with SPECS, but 40mph ALWAYS causes more accidents. I was a van driver for almost 2 years and I noticed that there's always more accidents when the limit is 40mph than 50mph.
Take the roadworks on the M25 near the A10; it's almost always nicer to drive through there now because the limit is 50mph. When the limit was 40mph, you'd have people going slightly left or right of their lane('s) because they were so paranoid that they'd watch their speedo as well as trying to watch the road.
Again, I don't mind roadworks and limits, as long as they're feasable. I hate...and I mean Hate adhering to 40mph or slower speed limits at roadworks where there are no workmen working at all.
As for HGV Tailgating...it's annoying, but if they hit you from behind, remember, it's their fault. If they want to get 3 points and a fine, it's their choice.