Well, the amount of code-switching she was doing was impressive as all hell. Not everyone can learn to switch their accent and their body language, and she had both absolutely pitch-perfect. I actually think that bit is more impressive than learning (or faking) a proper governess's knowledge base. But in the real world, people have managed that sort of social leap.

Also—speaking for myself personally—I don't mind if Doctor Who errs a bit on the side of hope, rather than historical accuracy. In the real Victorian world, Madame Vastra might have ended up in a cage long before she became indispensable to Scotland Yard, but I don't want to see that story. I like the one where she and Jenny have a good life and a solid relationship even if that relationship utterly fails to compute for ninety percent of the people they run across. Likewise, it might be more probable for a barmaid-becoming-governess to make some tiny error and get hauled away in disgrace, but I like the version where cleverness and nerve prevail. I mean, there's a believability limit, obviously, but I didn't feel like the episode was ignoring historical realities so much as suggesting that it is possible to break free of your "station," and always has been—even when society is built specifically to prevent that sort of thing.