The law is quite clear, no driving on motorways with a provisional licence. If you allowed learners on motorways, you could end up with people driving on motorways with only a couple of hours of experience.
I've driven with a learner on A roads like the A1 and A2, which are both dual carriage ways, which is allowed. When they turn into motorways like the "A1 (M)" and the M2, you have to swap drivers.
For example when driving south on the A1 towards London, it becomes a motorway just after Peterborough, but there are services at Peterborough, so you can pull off and swap drivers.
Driving on a dual carriageway is plenty for a learner to deal with without dealing getting onto a motorway. If they want experience of a 3 lane road before the test there are plenty of places to do that without being on a motorway.
If you want to drive "around" London with a learner, you'll have to head to the North/South Circular.