Whilst I agree it seems to be assault in this case, it certainly wouldn't be in every case. A defence to assault by driving a 2 tonne vehicle at someone with the intention of nudging them so that they refrain from committing an offence of obstruction would be that the force used was necessary and reasonable in the circumstances for the defence of another or prevention of the crime.
It would be quite unlikely for a court to regard nudging as excessive force if there was clearly no intention to injure, so in most cases it would depend on whether the action was necessary. For a child being late to school, probably not, but if someone in the vehicle had a relevant medical condition or was visiting a seriously ill child in hospital or whatever, the court could easily decide that the defendant honestly beleived that the course of action they took was reasonable and necessary.