Wow, lots of responses! I love you all.
I should mention that I have taken enough Spanish classes that I ought to be fluent in it, I've just never needed to use it, and so it has gone rusty. I could pick it up again fairly quickly, and at this point could probably read something in Spanish, though slowly and uncomfortably. I have a decent head for languages, and I don't feel like making the leap to Latin or French from this point would be a step too far, given some time and effort. Is this something I should be concerned with at this point in the application process? Will it be held against me (or rather, to what extent will it be held against me?) if I'm functionally monolingual when submitting my applications? The impression I got from perusing the sites of various departments is that a certain amount of language skill will be necessary at some point as one proceeds through the PhD program, but they usually aren't super clear on how far such things get taken into consideration during admissions.
I've looked into master's options, and it might make sense as a bridge, given that I'm coming from a non-history undergrad background. But given that what I want out of the whole thing (a career poring over ancient musty tomes, one hopes!) requires a PhD in the end anyway, and considering that master's programs are typically not funded (more student loans? ohpleaseno), I'm not sure that's a route I want to pursue, unless it's definitely the best way forward.
Also, I don't suppose anyone here has experience with the whole applying-to-schools-in-the-UK-from-the-US bit? It's definitely an interesting option given the area I want to focus.