About the best explanation I have found online is this one:-

Learner drivers allowed on UK motorways?

As a driving instructor, I personally would welcome the change. To give pupils experience of dual carriageways, I have to take them on the non-motorway section of the A1, which is frequently two solid lanes of traffic with few spaces for a learner to join. There often is no way that traffic in the inside lane can move over, as the outside lane is also solid traffic.
By contrast, the M62 in the direction of Hull is relatively quiet by comparison, has three lanes for traffic to pass and much longer slip roads. A heck of a lot safer for driving on.

Quack!
GtD.