The PM's announcement struck the wrong note on a number of fronts. It made it easy for Labour politician Douglas Alexander to attack him as "typically arrogant". The U.K. population has historical form for disliking leaders who look like they are assuming the results of an election. The best-known example is probably the 'Sheffield rally' – when Neil Kinnock, then leader of the Labour Party (and now better-known as the father-in-law of the Danish Prime Minister Helle Thorning-Schmidt) appeared to enthusiastically celebrate an expected Labour victory, only to lose the election a week later.
Another is Cameron's predecessor Tony Blair, who famously did not make it to the end of his third term after promising not to serve a fourth term – turning himself into a lame duck.