The rules to be 'legal' are such a rats nest. When I bought my conversion, I was under the impression that converted bikes that were older than 2015 had grandfather rights to be able to have a throttle as well as PAS. I saw on Fleck's thread about the law today, that this is not really true and that conversions need to either carry a plate from the manufacturer, or go through a test. Everybody you speak to comes up with a different answer - even people in the trade. Back in May, I was in the Lake District talking to a bike hire company, wanting to try riding a pedelec before deciding to buy or not, and the owner of the business was showing me 750 watt ebikes that he had for hire and telling me I could ride them on the road. When I told him the law had a limit of 250 watts, he got angry, actually told me I was talking 'sh**e' and told me to go somewhere else.... He was a proper charmer and no mistake.
It's a good job that the police have too much to do to be pestering sensible riders about their ebike conversions, because most of them won't pass muster according to Fleck's remarks. However, since they don't even investigate burglaries or fraud anymore, and hand out crime numbers and victim support telephone numbers instead, they are probably not that likely to be bothering people on bicycles who are riding sensibly on the roads. Those riding like maniacs are in a different category I expect.
My Bafang throttle is a good safety feature actually. It is great to get started on a hill. I rarely use it and never ride the bike like a moped without pedalling. If I want to do that, I ride one of my motorbikes.