I believe they set the motorway speed limits back in the 1960's and we all know that today's cars are a lot different to the cars back then.
Safety has greatly improved, not to mention stopping distances as well as performance.
Imo we should be concentrating on becoming more like the German Autobahns and improving everyone's driving standards.
Generally people on forums like these and car enthusiasts in general have a good grasp of what their cars are capable of in terms of braking/stopping distances.
Yet the majority of users on the road seem to have no idea of these facts as anyone who drives daily on the roads will know as we've all been tailgated by idiot drivers who seem to think their cars are capable of stopping within a metre or two at motorway speeds of the car in front of them.
Sometimes to me it even seems that some people get worse when the weather conditions are dire as because then there is even more tailgating going on.
Personally I'd like to see no speed limits on motorways and people using more common sense when driving (driving to the conditions, traffic etc), ...but unfortunately it will never happen as common sense doesn't seem to be so common nowadays.