E-Bikes - Why can't I have a full electric mode?

The legal definition of an Electrically Assisted Pedal Cycle (EAPC) or E-Bike is an electrically assisted bike where an additional motor is fitted to supplement the cyclist. Fitting a bike with a full electric mode or a throttle changes the definition of the bike to one of an electrically-powered motorbike, where a license, insurance, road duty and MOTs would be required.