There were a series of British commanders in the Napoleonic Wars, which lasted a long time and were scattered across the world. For much of the conflict, the overall commander was the Duke of York, but by far the most significant British commanding officer, and the one who was in charge of the British forces in Europe at the end of the war, was Arthur Wellsley, who became the Duke of Wellington after his services in the Peninsular War. Wellington commanded the British forces, as well as Iberian fighters in that conflict, which proved to be a disaster for the French, to the point where historians have called the campaigns Napoleon's "Spanish ulcer."
After Napoleon's catastrophe in Russia, Wellington's forces drove the French out of Spain altogether, and even launched an invasion of France, during which he was wounded. He became the Duke of Wellington upon Napoleon's surrender and exile, and was the leader of the British delegation to the Congress of Vienna. When Napoleon returned from Elba, Wellington took charge of the British and Hanoverian forces that eventually defeated Napoleon at Waterloo.