IIRC the 70 mph and 60 mph "National Speed limits" were introduced as a fuel saving measure during the oil crisis, before that de-restricted roads were just that - not limited.
The 60mph roads around here are rapidly becoming 50 mph too - didn't anyone realise that adds 20% to journey times if you are lucky and 30% if the duffers who won't go near the limit are only doing 40
I remember Eric Broadly at Lola saying that in the 60's they road tested the T70's on the M1 but had to keep below 100 if there was other traffic because morris minors were only doing 40