With the speed limit at 70mph (on M/ways) you hardly ever see people doing 70 - most repmobiles do 85-90. So raising the speed limit to, lets say, 80mph would mean that all those people will now do 95-100 and think nothing of it.
People drive like spanners as it is so I would say its fast enough. I also agree with Ekona, they have this system in France on all m/way signage.