The Union Jack came into widespread use during the rise of the British Empire, which needed a flag to plant on all those bits of land, not to mention to wave at coronations, victory parades and such. Then came the decline of the Empire, Beatlemania, the Swinging Sixties, Mod (and several rehashings thereof), punk rock, neo-Nazi skinheads, Britpop, Cool Brittannia, New Labour, the Spice Girls and the Austin Powers movies, by when the flag had become a successful lifestyle brand, an example of what the pundits call "soft power". Somehow, in the interim, nobody actually remembered to pass a law enshrining it as a national flag, and hence its status is more a result of habit and precedent than anything more formal, which does seem like an appropriately British way to go about things.
Some other pieces of Union Jack/Flag trivia:
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