White cockades would not have been worn by British regiments as it would be considered treasonous.
The hackle is worn by those regiments that participated in Paoli's Tavern during the AWI. The Colonials swore revenge so the regiments put a hackle of red feathers in their tricornes so they would know who did it.
The Black Brunswickers wore black as a sign of mourning for their lost duchy.
The Bonny Light Horseman, like Over the Hills, predates the Napoleonic Wars.
Hobbitmum, maybe make him an observer with the Russians or Prussians?