As you've now gathered, you have two humps to get over - speed limit and motor power.
Speed limit is fairly straightforward, the motor cannot assist above 15.5mph, although there is a measurement tolerance of an mph or two.
If you set the limit to, say, 16.5mph, you would be legal from that point of view.
Motor rating is a grey area, not least because all motors can deliver far more peak power than their nominal rating.
But the only figure in the regulations is 250w, described as a maximum.
You would be riding a bike with a published maker's spec of 1,000w.
So straight away you are very much on the back foot, if not obviously illegal.