That makes sense really. The trouble with all these laws are that they are to some degree open to interpretation by the man you meet on the day. If you get a friendly fella who just wants to do an honest days work then you're most likely fine, but stumble across a little weed on a power trip and you've had it. They'll find something if it takes them all day!
For motorway use I suppose you should be registered as a light truck rather than a tractor? When I get one maybe I should put it through a mock MOT? For my own peace of mind for one, but in another way it could tell me how much it's likely to cost to run as an LGV.
I'm starting to think that way because for a few hundred quid a year it's going to give peace of mind when I'm taking a load of gravel to do a customers drive.