No one's going to do this hypothetical scheme so that they can legally drive at 80mph (they may do it to improve their driving with no intention of going quicker than the limits, but that's a different thing)! The point would be to be doing 150mph+ where appropriate.
It is of course true that there are diminishing returns with every extra 10mph, but on a long distance journey on decent roads you'd certainly be getting useful ones up to 200mph at least.
There's also myriad other advantages like being able to exceed the limit to overtake, which suddenly makes a lot more 55mph dawdler overtakes plausible. Not to mention just being taught to overtake - this is something that the IAM or whatever currently can teach you but there'd be more incentive for drivers to actually learn it; you rarely see a proper roadcraft style overtake out on the roads*, a tailgate lunge and hope style seems to be far more common.
The German car industry has benefited a lot from the autobahn and how it's ended up with cars that are completely effortless and drama free at speeds that provoke a lot of pearl clutching in this country; if we were to join them there along with other stuff to encourage the EV industry (which has trickier problems at a sustained genuinely high speed for any distance at the moment as you'll give back everything saved in extra charging) it would be good for us. Will never happen of course, as neither will my dreams of massive motorways everywhere, bringing back the London ringways, etc etc... but that is what it is.
*Here's a beautiful example of how to overtake properly by a trained driver, called by the author "worst overtake I've ever seen", which just goes to show how little understanding most drivers have and also how annoying dashcam warriors are:
There's no realistic chance of having any driving speed limits of over 100mph.
First up our roads are too busy, there would be so few chances to travel at over 90mph that it's just not worth the change.
The biggest barrier however is likely the fact that other traffic would just not be competent enough to allow for such high speeds to be used.
Don't believe me, how many times have you seen a car on a road with more than one lane per direction not read the road ahead and end up braking behind a lorry. If their driving skill is so poor that they do that for a vehicle with a (say) 15mph speed difference in front and is much larger than them, what's the chances that they'll not notice a smaller vehicle approaching at speed from behind and pull across into their path?
Now to combat that the advanced driver would need to slow down to ensure they have time to react if that were to happen.
As such, even if the road was lightly enough used that there was only traffic in the left lane, for a significant amount of time the advanced driver is going to be gong much closer in speed to the existing limits than the higher speeds.
Likewise at each junction, there's a chance that two cars join and the rear one pulls out after merging to overtake the front vehicle, so again the advanced driver would need to slow down.
It would also only be possible to do so on roads the driver used fairly frequently, as at 120mph they would need to be aware of around 55m of road every second. That includes being aware of changes in camber, debris, surface defects, what's happening 50m in front of the vehicle (1 second away), 100m in front of the vehicle (2 seconds away), 400m in front of the vehicle (highway code stopping distance would be about this far) and over 1km out (20 seconds away).
Even then, the time savings would be fairly limited. Averaging 90mph rather than 70mph would mean a 180 miles of motorway driving (which amounts to a few journeys a year for 0 million drivers) might save around 30 minutes, however that's likely to be on a journey of over 3 hours.
Also I assume that is not a pass once test and you're good for the rest of your time driving, so that's probably 90 minutes every 3 years just for the assessment (could be more depending on journey time to get to the assessment), so overall few withs actually save time.
So that's a load of admin to save 0 million drivers 0 days of time.
The bigger time saving for drivers would be if the number of miles travelled dropped by 10% as that would significantly reduce congestion and most likely onthe journeys they spend the most time doing.