The safety issue is concerned less with the spped limit on a motorway, than the different driving skills required, which require a good deal of experience before being attempted. The potential for bottle-necks or catastrophic pile-ups which could result from inexperienced learners on the motorways is hopefully minimised, although any learner is able to zoom off down a motorway five minutes after passing their test, so maybe the rule is not as clever as the MOT would like to think!