Surely JOM is our resident expert on this subject....... 
Hardly an expert, but as we are having "managed motorways" on sections of the M3 and M4 soon, I have attended meetings on these over the past couple of years when they were starting to implement schemes elsewhere including this one, nothing was ever mentioned of reducing the maximum speed limit then
The section involved runs through seven junctions which are well spaced apart, so few areas of conflict as people leave/join. Because it’s “managed motorway”, when permitted, you can run the hard shoulder when congestion is present, and overhead signals can be changed to give varying speed limits dependant on traffic volumes at that time. By all means reduce the limit to 60, 50, 40, 30, when congestion occurs, but there will be many times (I daresay a high proportion of the time) when 70 is an appropriate speed limit and should be permitted
As technology improves, and pollution from vehicles decreases, will they then raise it back to 70mph? I very much doubt it. If this goes through, it will be the thin end of the wedge, very soon we will see other sections of motorway having mandatory 60 limits applied because of “pollution”
In short, the proposal is utter bollocks