Although the Treaty of Ghent had been signed the previous December, it wasn't ratified by the USA until 16 Febuary 1815.

A main contributor to the British defeat was the actions of the officer commanding the 1st Battalion, 44th Regiment of Foot (East Essex). He was supposed to bring forward fascines (big bundles of brushwood) and scaling ladders to deal with the canal and earthworks. So when the attacking infantry reached these obstacles they got stuck there. details here Of course, there was no guarantee that would have meant success. But the some of the British officers had convinced Pakenham that American militia would not withstand a strong frontal assault (viz Bladensburg).

Incidently, the 44th was the most-massacred English Regiment - at Prestonpans by the Scots, by French and Indians on the Monogahela River and by Afghans in the retreat from Kabul, 1840. Just the lads you need in a tight spot!