Why Britain reinforced Canada during the US Civil War

Was Britain building up forces in Canada in case the American Civil War widened?

Yes; Britain reinforced Canada because some US politicians considered war with Britain as a way to reunite the country against an external enemy.

Britain did build up forces in Canada during the American Civil War. Some American politicians discussed conflict with Britain as a way to redirect domestic division toward a foreign enemy. British planners took the possibility seriously enough to reinforce the colony and watch events closely, though invasion was not treated as inevitable. The civil war therefore generated external-security calculations far beyond its main battlefields.