@Perfectos - you may disagree but Ive been going up & down the M6 for years,
Our works vans , T6 van with driver = van charge, van with 3 up front = car charge.
Our T5.1 cali used to be hit & miss as to what it was charged at when loaded.
The only consistency has been always being charged as a van when theres only a driver.
The height is measured supposedly at the centre line of the front axle, because of the steep angle of the back end of the bonnet / bottom of the screen it makes a big difference exactly where the height is measured.
The back edge of the bonnet on a standard van is 1.26m which is just in front of the measurement point, measure 150mm further back & the height of the windscreen at that point is more like 1.6m
The height of the rear suspension also has a big effect. A drop of 100mm on the rear effectively moves the the top edge of the windscreen back by about 66mm.
The van is so close to the Tolls measurement divide that it just depends how you are loaded. You only have to look at previous threads re the overall height of vans to see the differences in height of what are supposedly identical vehicles when new.