As an armchair historian, my educated guess is that it has to do largely with British influence and occupation in the Middle East, Far East, and India. But in an imperial society like Britain's was/is, how did a foreign "delicacy" became a staple of the nation doing the colonizing? How did tea become standard in the United Kingdom?

Consequently, in reference to the second question, I happened to be watching Series 2, Episode 2 of BBC's Sherlock and overheard several references to fixing coffee. In the U.S. at least, coffee is the typical drink and social convention. To what degree is coffee a staple in British society? Is it competing or replacing tea to any degree?