Depends on the power output apparently.
This is the current situation
UK Electric Bike Law
The current situation: At present, all electric bicycles sold in the UK should comply with the EAPC (Electrically Assisted Pedal Cycle) Regulations. These say that an electric bicycle is classed as a bicycle if the motor will not provide any power above 15mph, and that the motor must have a continuous output of no more than 200W. All cycles which fall within these limits are legally treated as bicycles - you can ride them without a license, and they don't need insurance, road tax or a helmet. Any cycles which exceed these limits are classed as mopeds, and must be type-certified, insured, taxed, etc.
So for this type of electric bicycle the only requirement is that the rider be over 14.
Worryingly there is also no requirement to wear a safety helmet