I wouldn't say he was a "complete failure". Won a general election, never lost one. As a party leader, that doesn't mark him as a "complete failure". In British politics, the vast majority of Prime Ministers do not end their tenure in their terms. In the 20th Century, on Stanley Baldwin. Churchill and Wilson went out due to ill health. Blair planned to stand down, but it was his party that forced him from office. Party leaders have one goal to measure success; winning a general election. Foot, Kinnock, Hague, Howard, Duncan-Smith, Miliband and Magic Grandpa (Corbyn) were all utter failures.