The throttle can work up to 15.5 mph as long as power stops when the pedals stop. The 250w max is the rated power, not the actual power. The actual power can be whatever you want as long as you use a 250w motor.The uk limit for an e-bike or e-scooter on a uk road is 15.5 mph. Motor must be 250watt max with the thumb throttle on a pedal assist only able to accelerate up to 4mph.
Anything greater than that on a uk road you risk getting done.
The hub is supposed to have the wattage shown on it by the manufacturer.
loads of info on this at gov.uk
Bobbo.