I remember it well. Even in the '70s, most cars that ordinary people owned wouldn't easily go at speeds over 70 continuously so the 50 limit was broadly kept to (certainly most would go less than 60 on open roads. I didn't live near any motorways so can't comment on what happened on them. The big differences now are that almost any car or van can happily cruise at 70 for miles and there is a sub-culture of speeding by some self-entitled drivers, (often justified by some bad science about how their car/driving style is actually less polluting and they are much more alert and less likely to be involved in incidents at illegal speeds), so any attempt to reduce actual speeds would need to be backed-up with comprehensive enforcement measures.
One good measure is that electric cars are not only zero emission at the poinnt of use, but also, when running at sustained high speeds, the reduced range will be more apparent, which might help teach those drivers to moderate their behaviour. If they think that they can avoid such limitations by sticking with polluting IC engined vehicles, the ever increasing cost penalties of such vehicles versus electric will at least give everybody else a return.