From the government website, depends on if it’s registered as a ‘electrically assisted pedal cycles’ (EAPC) or a motorcycle.
EAPC must have a maximum power output of 250 watts
should not be able to propel the bike when it’s travelling more than 15.5mph
An EAPC can have more than 2 wheels (for example, a tricycle).
Where you can ride
If a bike meets the EAPC requirements it’s classed as a normal pedal bike. This means you can ride it on cycle paths and anywhere else pedal bikes are allowed.
Other kinds of electric bike
Any electric bike that does not meet the EAPC rules is classed as a motorcycle or moped and needs to be registered and taxed. You’ll need a driving licence to ride one and you must wear a crash helmet.