There are many parallels with other products that have been sold even though using them could be against the law.
We've already seen that small electric scooters (those without any form of approval), small "mini-moto" motorcycles, Go-Peds etc have been sold here in some numbers before the law clamped down on them. The police are now reasonably savvy about the legality of them, so tend to act.
My guess is that it will take some time before the police become similarly savvy about the illegality of the Optibike (when ridden on a public highway, which includes byways, bridleways and any track, path or road to which the public have a free right of use).
One side effect, assuming that Optibikes sell well (not too likely, at the price, I think) may be that some marginally legal ebikes get subjected to police scrutiny. Knowing the way that we get knee-jerk reactions to things, it wouldn't surprise me to find something like this creating a push to ban throttles on all ebikes, even retrospectively. Given the increasing interest in high-powered ebikes, and the publicity on You Tube etc showing the performance of some of them, this may well happen anyway.
Our government is not known for acting rationally and in proportion to risk in things like this, especially if someone is unlucky enough to get hurt by an illegal ebike. We all know that 99.999% of gun crimes involve illegally acquired guns, yet that didn't stop the government deciding to effectively ban legally held sporting hand guns. The inconvenience and cost to sporting gun enthusiasts hasn't been balanced by a reduction in gun crime at all, more's the pity. In fact we have many more shootings now than we did before the law changed.
Jeremy
PS: I'm not a sporting gun enthusiast, BTW, I was just using it as an example of poor law, made as a consequence of a knee-jerk reaction to a madman's actions.