With all the Smart-Motorway systems, it’s easily controlled.
I wouldn’t consider 85mph at quiet times particularly fast, in any modern vehicle.
The biggest stumbling block, is driver etiquette.
I do envy French and German motorway systems. Maybe it’s time for the tolls…
I once (7 years ago) had the pleasure of attending a speed awareness course which actually found quite interesting.
The bloke running the course explained that a motorway is generally the safest place to have an accident as the flow is in one direction with good visibility, barrier that will stop you from flipping and a hard shoulder to run out on (apart from smart motorways, depending on how fast the info is updated).
Issue is that your car might be well maintained, capable of stopping quickly…but the person behind might have an older car that can't stop as quickly, so if you hit the anchors hard they might well end up in your backend, so they have try to set a speed limit that suits all, or, have a variable limit that changes with the weather…but this costs a lot of a lot of £ to implement.
A-roads can be more dangerous as even under the speed limit…say 50 in a 60. If you hit a car coming the other way that's potentially a 100mph impact. Hence its important that cars are well maintained with tyres rotated to ensure deepest tread is on the rear (2wd), as its easier to control a front end slide as will go into the hedge rather than loosing the back end and spinning. Make sure that you don't fit seat covers that prevent side airbags from functioning correctly.
In 20/30 mph zones. If someone steps out from between parked cars and you hit / kill them. If its proved that you were exceeding the speed limit then the penalty can be very severe even though it wasn't your fault. Might lose your license, depending on your profession might lose your job…then you can't pay the mortgage etc.
Also said it was better to drive (manual) in a lower gear with higher revs in a built up area as your engine braking was far more efficient versus driving in a higher gear.
Most consider themselves as safe drivers, but you can't account for others on the road. We all all probably guilty of bending the rules from time to time…