This is one of those stories that is embedded in our culture to a sufficient extent that it may now be impossible to appreciate it as it was intended. I am pretty sure, for example, that for the first half of the book one is not supposed to know the identity of the Scarlet Pimpernel. And when the Pimpernel turns up in various disguises, one is not supposed to know it's him until the reveal. It all seemed absurdly obvious to me as I was reading it, so I was never taken by surprise in the way that the author might have intended.
On the other hand, there are certain scenes that are quite affecting if you know who the Pimpernel is, but if you don't know, those scenes lose a lot of their punch. So perhaps Orczy intended the reader to see through everything right away.
For the scenes set in France, Orczy consistently gives short distances in meters. I don't think the meter was in common use in England in 1905, so I think this was intended to give the narrative a post-Revolutionary French local color. Unfortunately, it is an anachronism: the story takes place over about two or three weeks in September and October of 1792, and the project to establish the meter was only just getting started then, so it is unlikely that ordinary French people would have been comfortable with it yet.