What made me think it was British? At first I think it was largely my untrained intuition. I could make a more convincing argument based on the subject matter though. I have come across several British ballads dealing with love, but few American ones. This is not to say that American ballads do not deal with love at all, but it's rarely the central problem of the text. A look through Laws' classification might be able to confirm this. There is no category entitled "Ballads of Love" in Native American Balladry, but in American Balladry from British Broadsides, there are four categories dealing with love. American ballads about love are relegated to the other categories. Of the two that come to mind, "Lost Jimmy Whalen" is in the lumberjack category and "The Plain Golden Band" goes in miscellaneous. I should note that there is also a debate about whether or not "Lost Jimmy Whalen" is actually of American origin. No British original has been found, but that hasn't stopped scholars from speculating that its author had a British template. And, as I write this, I'm wondering if "The Plain Golden Band" has only escaped that fate because the evidence is so compelling that it was written by a man from New Brunswick. Maybe if we weren't pretty certain it was the work of Joe Scott, people would look for its British original. I have no doubt that "Pretty Polly of Topsham" itself was composed in the US, but I wonder if it was reworked from a British Ballad.
Of course, there are all kinds of holes which can be poked in this argument about differentiating subject matter. One could probably make an argument that several American murder ballads have romantic love as the central problem of the story. And "Plain Golden Band" goes against my theory. After all, just because there are few American ballads about love doesn't mean there aren't any.