Fair enough. One of my paranoias was specifically meeting a police officer who is an ebike enthusiast. Or not even an enthusiast level of interest would be necessary - after a couple of days of internet research I was aware that all of those large-diameter hub motors are overpowered with respect to the legal limit. I was worried that after some twat inevitably rides a 1000w bike too fast and hurts someone, all police will be issued with these guidelines of how to spot illegal bikes easily, as part of a crackdown. The Bafang you've got is much more discreet looking than the ebay 1000w hub motors.Unless an officer is an ebike enthusiast they will never spot if the motor is overpowered or not. Event then, unless there is a label on it that says it's more than 250W, or there is no label at all (as required by the law), you could claim this is a 250W kit. Similarly with mid-drive motors, it would be difficult to know for certain if it's a 250W road legal or 1000W overpowered motor just by looking at it if there was no label on it. Riding fast does not necessarily mean the motor is overpowered, you could have just been pedalling really fast. Central London is full of cyclists hitting 30-35mph on road bikes.
The only way to prove you wrong would be to take your bike to a testing facility which is extremely unlikely unless you've been stopped for doing 100mph; routine check would most likely be limited to a conversation. Be a nice guy and you'll just be told to slow down.