Cars may be better these days - but the drivers aren't. Driving faster requires more attention, and no distractions such as the stereo, passengers, let alone phones. Are your tyres up to it? Or a different tyre on every wheel? Then add the fact that in the dark 70mph really is quite fast enough, throw in some rain, 1.6mm legal minimum tread...
People then talk about enforcing it more closely. But safe traffic flow requires a level of speed disparity between vehicles, to reduce bunching and tailgating. You actually want a fairly loose interpretation of the speed limit. We're all aware off the effect of one truck passing another when they're both sat on their speed limiters, or bunching in average-speed-camera zones.
Neither would it help our CO2 emissions. And the government said it would increase productivity! How? The few minutes saved would be at either end of the day, you'd spend longer in bed instead.