I think it's more about getting people confident at speed _first_ on slightly less intimidating roads - and for that a 70mph dual carriageway will suffice rather nicely. Around here it makes little difference in reality as the A14 is 3 lanes in parts anyway and there's only one motorway in the region (M11) which is two lanes.. There's nothing to stop you taking your advanced driving test or even just taking a couple of lessons on a motorway post passing your test.
Motorways require a lot more awareness than other roads - the reality is people travelling at far higher speeds (the M1 near leicester is three figures in the outside lane regularly) over _and_ under-taking, more agression and far less patience from other drivers (do 70 and people swear at you as a learner for holding them up).