I found out about my ancestor being in the War of 1812 from the Musters of his regiment which can be found at Kew in the National Archives. They may be available online now.
Thousands of civilians had been recruited to the army and navy for the Napoleonic War in Europe from 1803-1815. Try 'Find my Past' for Napoleonic Reservists and recruits.
My ancester had been in Portugal and Spain in the Napoleonic War from 1803. An expedition was sent from Bordeaux in 1814 with soldiers who had been fighting in Portugal, Spain and France. He went to Chambly in Canada (near Montreal) and returned to France in July 1815. His wife travelled with him and she gave birth on the way to a daughter (at sea registered as Guadaloupe).