Name a random country, and there's a pretty good chance that we've sent an army there to kick fuck out of the locals and make off with their valuables. This meant that a lot of young lads who might otherwise have misspent their youth thieving on the streets of the Gorbals were instead sent out to beat some healthy respect into Johnny Foreigner.
The list of nations with whom the UK has picked fights is truly sensational, encompassing most of the globe, and even the most misty-eyed Imperialist would have to admit that most of these weren't exactly defensive actions.
I, for one, can't remember reading about young Winston dodging assegai during a Zulu assault on London, or deprivations caused by a Chinese blockade of Portsmouth.
This speaks to a seriously high level of aggression in the British character, as our former enemies can attest. Reading about Britain in the 19th century is a little like reading a version of The Lord of the Rings in which Tolkien dresses his orcs in starched uniforms and has them witter about the benefits of civilisation before they burn the Westfold.
Obviously, there is more to it than this (as The Flying Rodent acknowledges) but I think that his point is, actually, a serious one.
People were poor in the 1800s (though getting richer all the time) and one of the ways to get out of the gutter—and avoid the workhouse—was to join the British Army. And our territorial expansion meant that there was an ever-increasing demand for men to fight overseas. Further, the rewards were potentially huge: not only because one wasn't unpicking hemp for a bowl of gruel a night, but because the loot would also be added to your salary. Many soldiers went away destitute and came back very rich men (those who did come back, anyway).
Further, of course, people who had been at the very bottom of the heap in Britain suddenly found themselves part of the ruling class in the colonies. OK, they may not have been at officer level, but they were definitely ranked above the natives. This is, I imagine, one of the reasons why so many British soldiers became ex-pats and effectively settled in the countries in which they had been stationed.
Those people who demand that the young of today "need a good spell in the army to teach 'em some discipline" tend to forget that we simply don't need the kind of forces that we did under the Empire; further, of course, the potential rewards are nowhere near as great.