Useful websites
If your ancestor fought in a high-profile battle these sources may help
Internet Archive archive.org/details/waterloorollcall00daltuoft
Waterloo Roll Call, a list of almost 4,000 men (mostly officers) who took part in the famous 1815 battle that defeated Napoleon.
The Peninsular War www.peninsularwar.org/penwar_e.htm
Useful background with details of the battles of Britain’s main campaign against Napoleon in Spain and Portugal (1808–14).