I had a caravan for several years, took it round France as well as the UK, always used a large Volvo estate for towing and could easily maintain 60 mph plus up or down hill, and having worked on farms for many years was fully conversant with the skills required for towing. I agree that some sort of test should be applied to people that want to tow anything bigger than a wheelbarrow. More traffic hold ups are caused by HGV's that are speed limited, one doing 58 mph trying to overtake one doing 57 mph whilst at the same time tailgating the lorry in front.
And most of the news reports including the BBC used the incorrect phrase "restricted to the slow lane" there is no slow lane on a motorway, 70 mph is the limit in all lanes, the second, third and fourth lanes are for overtaking only.