There are two different types of electrically assisted bicycles:
1) An "electrically assisted pedal cycle" (or EAPC, or e-bike, or Pedelec) if: the bike has pedals that propel it; the electric motor does not assist you when you’re travelling more than 25 km/h (15.5mph); and the power does not exceed 250 watts.
Bicycles that meet these requirements (which affect two-wheeled bikes but also tandems and tricycles) can be ridden on any cycle paths and anywhere else that bikes are normally allowed.
In the UK you must be over 14 years old to ride an electric bike, but you don’t need a licence, nor do you need to register it or pay vehicle tax.
2) E-bikes that support the rider in excess of 25 km/h (15.5mph) or has a motor rating above 250w are called S-Pedelecs.
The e-bike will need to be registered, insured and taxed as L1e category road vehicles. To ride the bike you will need a driving licence, have to wear a motorcycle helmet, they cannot be ridden on cycle paths and must be approved by the DVLA.
For more information on the specific rules and regulations in your area, please contact your local DVSA (Driver and Vehicle Standards Agency)