To be honest there's only one race, the human race, although I think I know what's meant.
To which I'd have to say what sort of mixed race idea is meant as I think even if you take for example, say, the Scottish, they are a mix of a lot of different and diverse tribes or ethnicities within the british isles and even the french (the trade routes used to be horizontal comprising parts of the European mainland more than vertically within the UK/scotland, england and wales).
I saw a news story about the northern league in Italy and the way the right wing populists from there had evolved and they had some guys on talking the alt right talk about the UK and the threat of political islam, it was curious given that most of the same factions within the UK want as little to do with Italy as they do Islam, the interviewer on the scene was saying that he thought this was about keeping the UK and Europe "white", which they did not disagree with.
I just thought, hmm, that's weird, I've never really thought of the Italians as particularly "white", the same goes for most of the US that have ideas about "white supremacy", they dont always strike me as that particularly "white", not compared to here, I also dont think here is that "white" compared to parts of Scotland either.