Technically, it is not a defence: it is a question of whether the constitutive elements of the offence are proven at all.
I disagree. It does not in my view relate to safety, but to a question of public health.
Not the right authorising Act for TfL, although it doesn’t make much difference in this case.
If reasonableness is on the table then it might well be argued that an instruction to wear face coverings with little concrete evidence as to their effectiveness is not a reasonable one to give, especially if done by a layman.
You would have to be extremely brave to make an argument that this was at least not "related" to safety.
You're making a distinction between public health and safety - which really isn't there. You can improve the public health by introducing safety measures. It only has to be "safety related" - which clearly, by the very nature of what they are asking, I.e. To wear PPE, in the form of a mask, is clearly, in their view, and probably of many ordinary people, safety related.
It's a very weak argument, but one for another topic if you'd like to discuss elsewhere.
If you believe a court is currently going to rule that masks are ineffective, you're barking up the wrong tree entirely. In any event, the argument isn't whether masks are effective, it's whether in view of all of the circumstances, it's reasonable enough to expect people to wear them (who don't have an exemption). Even if they are mildly effective, most will view it was reasonable, even if undesirable.
Regardless, as I keep stating, this is purely theoretical, it's unlikely to be enforced to any real degree, and as advised earlier, it has apparently been withdrawn.
If anyone disagrees with Byelaw 12, the only way you're going to make your point is if you take it to court and find out. So you can think what you like, but anyone who suggests deliberately seeking to test this ought to consider that it's not straightforward whatsoever.