Driving on a motorway is far easier than driving on a really busy A-road.
For example, the A14 (which is just down the road from me) is one of the country's busiest A-roads, carrying loads of traffic to and from the biggest port in the country (Felixstowe). It's the extension of the M6 (with similar volumes of traffic) but with only two lanes for most of its length, far shorter slip roads and traffic entering and joining the main carriageway far more frequently.
If you can drive on the A14 at peak times, you can drive on any motorway in the country (which is probably fortunate, as we've not got a motorway within 50 miles of here, so learner drivers get all of their 'motorway' experience on the A14 anyway).