Therefore, can I get away with it if "In use", I only engaged the 15.5mph limited throttle after I started pedalling, and disengaged the throttle shortly before ceasing pedalling? It's illegal according to the letter of the law, but might not be in it's "Spirit". But I'll leave my throttle disconnected for now, despite all the agony from my damaged and aged knees while starting at lights, or after a sudden stop at high gear, or on very steep hills etc.
I doubt you need worry about this law on throttles since it clearly isn't being policed. The DfT only introduced the change in the law in 2015 to tidy up the old anomaly on power ratings, so while they were about it they decided the easiest thing to do was to fully align with the EU law which banned fully acting throttles.
One year later we had the referendum which eventually took us out of the EU, a cruel irony!
In the interim period before departure, the DfT introduced a bureacratic way to allow throttles by going through a single vehicle approval test. That was in response to a consultation when we made it clear we wanted our throttles back. However, since that still breached EU law it was a rather silly move, but obviously they meant well to try to please us.
We can see that they are not in any way against us having throttles but have got themselves into a political trap. Despite Brexit, our politicians want to keep legal alignment with the EU on key factors like transport since it makes any future agreements so much easier to reach, so that makes it difficult for the DfT to reverse out of what they did in 2015.
Meanwhile the roads have thousands of e-bikes with throttles, some legally, some illegally and the police have no idea how to tell which is which. All that really matters is to have a 250 watt rated e-bike that cuts power at 15.5 mph, 17 mph in practice since the DfT have confirmed that the usual 10% tolerance is allowed. These two things are what the police can easily check so that's all they are likely to do.
So it's up to you whether you want to take a miniscule risk by using a throttle. In the unlikely event you did get pulled over and that is found, just act innocent and surprised, saying you had no idea it wasn't allowed. All that will happen then is they'll give you a warning to disconnect it since everything else about your machine is legal.
But again, even the chance of that happening is vanishingly small. The police really do have better things to do.
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