The rules are "Class 2 - Two axle vehicles less than 2.8 metres in height or three axle vehicles less than 2 metres in height" Hills M2 / Cost for travel on M2 and without checking I'm pretty sure the same applies to the M7 and other toll roads as well. So if you're getting the Class 2 toll consider yourself lucky.
The reason it's only 100KPH is so not to upset the push bikes in thedead lanecycleway, after all when you build a toll road you also have to build 4 cycleways that push bikes can use for free.

