This is why I think there should be tighter regulation on the minimum speed on the motorways.
While it's no substitute for correct observation and paying attention, you just don't expect certain vehicles to be going slowly. Nearly caught myself out with that one, came up on a car doing 40mph on a free flowing quiet motorway and had to brake. I was planning to move out but hadn't picked up on the speed of the car and because there was a car coming up in lane 2, I had to heavily brake.
You expect that large vehicles will be going slow, but not other traffic. Anything less than 60mph and you're not a commercial vehicle and you're a liability.