If it is an electrically assisted pedal cycle (i.e. it has pedals that can be used to propel it), it has a maximum power output of 250 watts and the electric motor won't assist you when you’re travelling more than 15.5mph then you don't need a licence to ride one and it does not need to be registered, taxed or insured. However, if it doesn't meet this criteria it is classed as a motorcycle or moped and you will need a driving licence, tax and insurance, and will need to wear a crash helmet. See: www.gov.uk/electric-bike-rules