You could always make the PoD, that Wellington gets sick just a little while before Waterloo. Hardly ASB territory. In those days even if you were rich and powerful, lots of stuff that's trivial for modern medicine could get you easily. Have him replaced by a real life version of Sir Henry Simmerson of Sharpe infamy and you end up with a British defeat easily enough.
The thing is, you have to look through the actual generals on the ground to see who replaces him. There aren't any Sir Henry's present. Not with the rank and experience to take command.
I mean, Daddy Hill may not be Wellington, but I think he would have been competent enough to keep the bulk of the army intact if forced back from the ridge. The French might draw a curse out of him, mind. And I think he'd get the gig. Or maybe Picton, who was also far to competent to be compared to a Simmerson. Harry Paget didn't have the battlefield experience to demand command of such a large force, thanks to cuckolding Wellington's brother, and while his heavy cavalry got smashed, he was still a Hell of a lot more of a soldier and officer than Sharpe's nemesis. And even if the Prince of Orange was as bad as Cornwell paints him (he wasn't), he wouldn't get command over the British Corps commanders, no matter his rank.
I think Hill and/or Picton can preserve the army, so it would be a withdrawal rather than Napoleon smashing the British (and allies) before rounding on the Prussians (then the Austrians, then the Russians, then...)
I think 'No Hundred Days' is the more interesting PoD.