Thought it was a motorbike initially..

[highlandsflyer]

That looks very much like it might belong to a guy I know!! He get literally everywhere on it!!
He has similar bikes for his two daughters, who are very often with him!!
I suppose,(in the eyes of the law), it IS INDEED,a "mechanically propelled vehicle; and,as such, would require a licence,insurance, MoT and payment of road tax/vehicle excise duty, to be valid on it; but, policing all that would be a nightmare!!!

The best way to 'police' these is to restrict availability, along with a better structured licensing/reporting scheme.