The throttle legislation is an unworkable mess and should never have been implemented at all as its ridiculous to prevent use of throttles when its the default power control for most ebikes in the world and provides maximum safety and control.
However if the ebike throttle is twist and go up to only 6km/h it is legal with regard EU regulations.
If the throttle only operates only when pedalling its legal up to 15.5mph assistance speed with regard EU regulations.
If the ebike was purchased before 2017 its still legal to use today twist and go throttle up to 15.5mph
As above posts if the bike is individually DVSA tested it is legal to have twist and go throttle up to 15.5mph.
If its an ebike kit according to the Department for transport its is legal according to this but this is more a grey area but the clarification did come from the Department for Transport.
https://www.pedelecs.co.uk/dft-pedal-cycles-converted-twist-go-exempt-type-approval/
What an utter waste of resources for the police to be confiscating any ebike with a throttle that operates at 15.5mph or below. Laws should be in the interest of consumers and safety not just some random nonsense someone makes up. There is no evidence to support any safety implication of having a working throttle on a ebike. It only goes to 15.5mph a fraction of the maximum speed of a bicycle unassisted on flats or downhill.
We really should be completely ignoring this EU nonsense and just allowing working throttles up to 15.5mph although personally I think it should be 20mph in line with our urban speed limit on many roads.
You wonder how the police are meant to remember all this nonsense that politicians and regulators have allowed. A simple no throttle assistance above 17mph (10% tolerance) is really the only test they should be doing and just making sure a working drivetrain with pedals is fitted.