I think that traffic and roadworks have a large part to play as well.
End of August bank holiday - London to South Devon - 160 miles in 6 hours - average speed less than 30! On roads where, if the speed limit had been followed, the average speed would have been closer to 60.
Last weekend - London to Manchester - close to 200 miles in 4 and a half hours. All of it on motorways where you might think you could do a steady 70... Sunday return would have been a nightmare if I had stayed on the motorway but ended up taking the same time along the A5...
B.