There's a lot of misunderstanding about the legality of ebike motors. You cannot restrict a 750w motor to 250w to make it legal. It's perfectly legal to run a 250w motor at 750 w though.The regulation is on the motor, not the way it's used.
Also, any bike limited to 250w max would be useless. Even the common or garden hub-motored bikes normally run at 630w max from the battery, which would be over 400w of peak output power.
Nobody is checking at the moment. Nobody seems to care. who knows if that'll change in the future? If they did check, the first thing would be the markings on the motor. You'd be in trouble if it says anything more than 250w. Without marking, you'd have to provide evidence that your motor is rated at 250w, which would be tricky because our Chinese friends write things like 250w-500w in their specifications. I guess that would be interpreted as 500w by any lawyer or engineer.
The only way that you can prove that your bike is legal is if it has an EN15194 certificate for the whole bike.