I sense those booing aren’t all racists. Just the most stupid, ill informed and impressionable amongst us.
It’s the same group of people who froth about everything else, anti-vaccine, anti lockdown, hard Brexit, BBC license fee, Meghan Markle, the Union Jack being on display.
We have an obsession with Great Britain and the Empire, it’s built into the national psyche. Influential figures take advantage of that and people become warped by it. Brexit and the desire to return to former glories is the most obvious example of it, but it manifests in other ways.
Ultimately some people become nationalists, and racist as a consequence. I think with some they get there easily and with many it’s a slower path, and perhaps it’s an unconscious bias than pure racism.