Best of all the electric bicycle legislation would allow anyone with half decent spannering skills to take say a KTM 125 sx field bike, ditch the engine etc and use the remainder as a donor for an E-mountainbike. Really very simple to construct. Providing it weighs no more than 60kg it can be classed as a tandem E-bicycle.
Not only do you get to do mental downhill runs (easy 40mph+), your also perfectly legal on ANY bridlepath, restricted byway or TRO'd BOAT/UCR...oh, and cycle tracks too. Plus there's no criminal offence of using one on a footpath as they are bicycles in law.