The geography of the UK makes it difficult to require that- there are very large areas where it wouldn't be possible for a learner to reach a motorway then get home in a typical 2 hour lesson. Even if you only required learners to go on a national speed limit dual carriageway it wouldn't be possible in some places (far north of Scotland, for instance).
A few weeks back I had a journey on the M42 at evening rush hour where the dynamic speed limit was frequently cycling between NSL -> 60 -> 50 -> 40 -> NSL. Each limit was enforced for 1-2 gantries.
Even to someone who understands the benefit of dynamic speed limits for ensuring good traffic flow, it's not a good experience.
[0] https://en.wikipedia.org/wiki/Motorway_Incident_Detection_an...
There's supposed to be an End or a national speed limit sign, but there wasn't.
That's the theory. However, based on my experiences, it seems the control loop is badly tuned. In general, people's paniced response to a new speed limit needs turning down. All that happens is that people randomly slow down or brake at each new speed limit, and 1/2 mile down the road, there's the regular traffic jam anyway.
However, I don't think everyone believes the limits are there for a good reason. Actual quote I once heard: "they're just playing silly buggers in their control room, ain't they, they don't know anything". Not helped by many other signs crying wolf about obstructions that often aren't there at all.
As for all-lane-running, casual observation of UK driving indicates to me that a blitz on driving standards[1] such as "keep left", "avoid making others brake" and "when you change lane or merge, it's your job to match your speed to the new lane"[2] would be a far quicker way to increase capacity and reduce accidents. And would cost substantially less than the billions spent on smart motorways.
[1]: especially as motorway driving isn't on the driving test and learners weren't allowed on motorways at all until 2018, so most drivers have never had any formal motorway training.
[2]: Not only cars: I see HGVs doing 58 diving into gaps in 70mph traffic (on two lane roads especially) and causing half a mile of brake lights and converting free flowing traffic into nose-to-tail queues every day.