I don't know about Waterloo, but Admiral Lord Horatio Nelson, during the naval battle of Trafalgar, had his flagship signal his fleet with flags that were deciphered as "England expects every man to do his duty." That was also during the Napoleonic wars.
To 'be British' was a call to be as brave as the heroes the English schoolboys in Queen Victoria's day learned about in their history books. The Victorians were committed to empire building.
I suppose it started with the Elizabetheans defending their isle (and the waters off Newfoundland) from the Spaniards (and fishermen from other nations) and getting in some piracy and slave trading while doing it. "If Spain had an empire, why not we English? After all, we trounced their Armada, God being on our side. So, we must be the best and bravest, and God has given to us the right and responsibility of governing everyone else."
By 1912, Great Britain had lost the United States (The Americans who fought were mostly of English and Scottish stock, so the English saw it as British defeating British), but they had Canada, Newfoundland, India, Australia, New Zealand, a few large chunks of Africa and a significant number of other islands and colonies, as well as influence in the Middle East. All due to British pluck and bravery. So the Edwardian Englishman was pretty proud of his "British race", and had to live up to his heroes and his bloodlines. As Bob Godfrey said, many Britishers considered themselves to be the epitome of all that was best in the human race.
Many Anglo-Americans and Anglo-Canadians saw themselves as such too. Other immigrants and their descendants were seen as inferior stock, useful for farming the prairies but not for governing the nations. Even lower class English were not really 'Grade A'. The slum child sent by Dr. Barnardo's Homes to work for an Ontario farmer was certainly not considered an empire builder (He was sent to 'decrease the surplus population' at home, as E. Scrooge once put it, and his employer or his employer's father might have been considered 'surplus' too, if he had not emigrated.); though the younger son of a gentleman or noble who received an allowance to set up as a gentleman farmer was supposed to live up to what his family and school had taught him because he was of 'the better classes'.