the current law does not stop you buying and riding a high powered electric motorbike.The current UK laws on this subject really need repealing. I can understand the reasons why a one eyed granny shouldn't do 40mph on a shared path, but for an average sensible cyclist on the road common sense should prevail.
Battery capacity, voltage or motor wattage shouldn't come into the equation anyway, that doesn't affect the speed chosen by the rider, merely the potential available.
I doubt if any cyclist, electric or not, could be successfully prosecuted for exceeding a speed limit as there is no requirement for a speedometer therefore no means of monitoring it.
Throttle control should also be legal not just to assist those with certain disabilities but what difference does having to peddle make anyway. In the early 1970's the laws were changed on mopeds removing the requirement of peddles and introducing a maximum design speed of 28mph. Maybe as pedelecs become more popular, there is a danger of new legislation being introduced to bring them into line with mopeds so they can be easily controlled?
You have to ride it on the roads, wear helmet and pay insurance.
The EAPC law is different, it's a concession to low powered e-bikes to be treated like normal push bikes.