They are not putting a speed limit on ebike, they are putting a speed limits on the power assist once it hit 15mph, there is a big difference between the two. The ebike can go beyond 15mph but it will have to be power by your muscle or gravity. As it hit 15mph the motor will have to shut down to comply with the law or else it needs to be registered as a motorized vehicle instead and requires registration and insurance.
legal ebike = ‘electrically assisted pedal cycles’
ebike which does not comply = 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.