Many Canadians and Maritimers did fight overseas during the British conflicts of the 19th century.

One notable example is Nova Scotian Victoria Cross winner William Hall, who served in the Royal Navy during the Crimean War and the Indian Rebellion of 1857. His experience was typical of that of many Nova Scotians that fought overseas in the sense that, rather than the Colony of Nova Scotia specifically sending a regiment to fight, Nova Scotians would individually join the British Army or Navy. Halifax even has a memorial to two Haligonians who fell during the Siege of Sevastopol ( Here are some pictures of the memorial).

There’s an article in the National Post about Canada's involvement in the Crimean War that you may find relevant. During the War the British pulled out many Garrison troops from places like Halifax, and Colonial Militias were formed to make up the deficit.

Your question though specifically makes mention of the Napoleonic Wars, and the answer is similar. Nova Scotians and those in other Colonies in British North America would sometimes enlist in either the Amy or the Navy. However, in Nova Scotia many were 'pressed' into British naval service against their will. Keith Mercer's Planters and Press Gangs: A Social History of Naval Impressment in Liverpool, Nova Scotia, 1759-1815 in The Nova Scotia Planters in the Atlantic World, 1759-1830 is a good survey of the subject. Most of the Sailors and Fisherman caught by the Press Gangs would serve in the North American Squadron, rather than off the Coast of Europe. Logistically it made the most sense, though even before war broke out with the Americans it became a necessity due to the high rates of desertion.

Desertion rates were high in part due to poor conditions onboard the ships, though being impressed against your will does not particularly endear one to service. In 1806-1807 for example, desertion rates in the North American Squadron ranged between 9% and 40% per ship. A year earlier impressment had even resulted in a riot in Halifax. According to Thomas Beamish Akins in his History of Halifax City : "Press warrants were granted by the Council on the 6th May (1805) to Vice Admiral Sir Andrew Mitchell, then in command of the station, for fourteen days. He afterwards demanded an extension of his warrant for six months, which was refused by the Council at their meeting on the 18th ... Mitchell, finding he could not prevail on the Council, undertook, in the following October, to send press gangs through the town without warrant. An armed party of sailors and marines from the Cleopatra, frigate, under the command of one or more officers, were sent out. The citizens resisted and a riot ensued, which resulted in the death of one person and the wounding of several others. One of these encounters occurred in the store of Messrs. Forsyth & Co., where a number of merchant sailors had secreted themselves. General Wentworth called a meeting of the Council on 23rd November, and it was ordered that the Solicitor-General should proceed to prosecute all persons belonging to the ships war who had been engaged in impressments."

Rather than join the Royal Navy, willingly or otherwise, many Nova Scotians preferred to enrich themselves by way of privateering. The Bank of Nova Scotia was even initially financed by prize money obtained during the War of 1812. Nova Scotia’s most successful Privateer was the Liverpool Packet, capturing 50 prizes during the war, was Captured by the Americans in 1813, then recaptured by the Royal Navy and was once again taking American prizes the following year.

Canadian volunteers were sent to fight in the Second Boer War (11 October 1899 – 31 May 1902)