This insignia depicts the archangel St. Michael defeating Satan during the War in Heaven. As with the majority of renaissance art and beyond, Satan has for the most part been depicted as a dark colour, be it brown, red, green, black.
When it was founded in 1818, I doubt King George IV said to his artists “hey lads, for the bantz, whack a picture of a white fella standing on a brown fella’s head on there for me will ya please?”
It’s depicting the “historical” (LOL) image as it would have been for portrayed for centuries before, not just in the UK, but elsewhere in the world. I don’t see this as a case of the UK/GB being “racist” as that tweet suggests. Because I reckon most people would look at that picture in context and say “oh look, that’s St. Michael defeating Satan”, rather than, “why is there a white lad standing on a black lad’s head?”.