Like most things, the justification changed over time. In the 16th century, it was all about the king trying to finance his massive spending habit.
Then it was about trade. Then it was about re-enforcing that trade. Then the east india company fucked up, and india because a colonial possession. Then the missions tried to convert the heathens.
after that I'm not sure. We don't teach empire at all. The problem is that nowadays its difficult to find readable history that isn't there to either totally damn or justify the empire. Even less so ones that draw on first party sources from the british and locals.