The historical detail in this clip is remarkable. Notice first the hundreds of candles being used to light the hall. In the days before electric lighting, that's exactly what it took to illuminate a large space like this. Also notice the thunderstorm and lightning raging outside the windows. It was this thunderstorm the night before the battle, that muddied the field. Causing a critical delay for Napoleon, that helped the Allies win the battle. And finally---the clothes and outfits, the jewelry on the women, is precisely the way upper-class British would have dressed at the time.
You give them a little too much credit. Period costumes are of course specified when you're doing something historical. (What do you expect? Patagonia vests?) You might be surprised that -- although this is an awesome movie -- there are a couple of boo-boos in this scene. First of all, the DoR's ball was held in a barn and not in an ornate hall like this. Second, in 1815 there were no pipes and drums bands ... that came later. This would have been all pipoes. Third, and pretty minor I'll admit, the quickstep tune is called "Cock o'the North" and it is a clear mistake because it wasn't adopted by the Gordons until the 1920s. None of this takes away from the epicness and thoughtfulness of this film.
Not really royal, and in all honesty records show it was a very "simple" ball, the Duchess of Richmon didn't have great facilities available, although no one knows exactly where the ball happened some theories put forward a coach house. The truth is the ball was historical and famous because of the people who attended it and the moment in which it was held, but the party itself was fairly simple for a ball of the time.
