This is just a theory.
But it seems like there is something more nefarious behind the Men of Letters rather than just some fussy Brits too stuck in their ways. Even if Dr. Hess does turn out to be "the big bad" (which would be totally lame) she made reference to the Elders, which is presumably a council of Men of Letters. I think there would be a certain satisfactory irony if it turned out that some supernatural force was pulling the strings in the BMOL. After all, killing all of the American hunters makes room for some nasty business to go down if monsters have no threat.