The UK and the US are two totally different markets. Traditional Publishing has known this for years and often have two editions of the same book, one for the US market and one for the UK market. This is to compensate for the slight spelling differences, but the books also have different covers and sometimes even the titles are different too.
Something about your book is hitting with UK readers and they are clicking then buying. My guess is that there's something about the cover and title is interesting them. It might also be worth considering whether or not to publish two versions, a US and UK version with different covers. (As self-publishers, we can do the same thing by limiting regional rights, or at least we could, I'm not sure if it's still possible to do considering that it's probably a loophole that someone exploited a hundred times over until Amazon finally got annoyed and just removed it for everyone.)