I've made this point before. I agree, the law strictly speaking says almost all e-bikes on the market are basically illegal, as it's possible to set them up to go faster than 25 kph. However, the law relating to e-bikes is a total mess, made up by people who don't seem to live in the real world and certainly can't keep up with rapidly changing technology.
It seems as if there is a law, it says nothing is legal, but we'll turn a blind eye if you don't make a nuisance of yourself.
At the end of the day, I think it comes down to what I call "plausible deniability of wrong doing".
If you fit a motor greater than 250W, your bike doesn't have pedals or it doesn't at least pay lip service to the 25kph limit, then you are clearly putting yourself in a bad legal position.
If you can show that you have tried to do everything reasonable to keep within the law, such as it is, I think you are more likely to get a sympathetic hearing.