I wasnt aware of all the precise details that Ozmacca had gone into (thanks mate, useful stuff!) but all I can add to what he's said is that I havent come across the Cockade used as a 'badge' to denote a particular Regiment as such.

I'd GUESS, & it is just a guess (& someone more knowledgeable than me might have a better explanation) that it MIGHT be the mark of a 'Chosen Man' (unfortunately the 'Sharpe' series has misled people to what that means). There were no Medals awarded prior to Waterloo, & think its entirely possible that prior to that, a soldier who performed an act of particular bravery, or skill might be 'decorated' by having a Cockade pinned to him, presumably at the discretion of the Colonel.

Less complimentary (& presumably less likely to be commemorated in song...*G*)...might be that someone who wore a cockade had been 'conned' into the Army by a Recruiting Sergeant, presumably won over by the cockades & ribbons & other 'fancy' additions to the Sergeants uniform to impress potential recruits... there is more than one instance of Recruiting Sergeants doing that kind of thing, & making the gullible recruit think he was going into the Army with some degree of favour (or Rank) compared to other new recruits.