The U.K. does have variable speed limits on most major motorways that change with conditions, congestion etc, with LED signs that display the current speed limit.
Smart motorways only adjust the speed limit down, and when there is no speed displayed then there are speed cameras every .5 miles or so enforcing 70mph
Smart motorways will readily adjust the speed limit down (often for no discernible reason), but they don't increase the speed limit were it may be appropriate e.g. good weather and very limited number of vehicles on the road.
Your post seems to imply that the reason 80mph isn't the limit is because of safety, but actually the governments main reasoning for not moving up to 80mph when this was last reviewed was actually about emissions rather than safety (last debated in 2011).
