im 43 bikenut, i have looked up the legal definition of a motor vehicle, and as you righty say its a self propelled vehicle, and the definition of self propelled is (Containing its own means of propulsion)
to which a motorized bicycle has no means of Containing its own means of propulsion. it has to be assisted.
you see the English police over here can be very bullyish to say the least, if one doesnt like you or the look of you and then they try everything in there power to arrest you,if all else fails then they use the public order act and get you that way.
i have had my bikes for a good while now, and i have recently bought one from dean at pipelyne, i think there a great idea, cheap to run and are very agile and can access all area's. the only downside about owning one in the uk is the weather as it rains more than its sunny. but still i use mine every day and plan to keep building them as and when i can afford too.