I take it you have never driven a HGV ?I've always been of the opinion that raising the speed limit to 70 for HGVs on dual carriageways and motorways would be better raising the speed limit for cars. If you speed up the left hand lane then it will improve the flow in the other 2 lanes as people are more likely to keep left.
The current speed limit and prosecution speeds are good enough, and as someone said above if you raise the limit people just drive slightly quicker than that limit.