It's my assumption that you are referencing the period of Imperialism before the decline of the British Empire, i.e. the Age of Imperialism.
The essential articulation of the colonizer's attitude can be found in Rudyard Kipling's "The White Man's Burden," which positions the colonial power as both the redeemer and civilizing influence of native populations.
There is debate over the specific distinction between the terms "colonialism" and "imperialism." It is my understanding that the distinction lies the fact that the imperialist program is one which is ideologically motivated. Simply put, the imperative is/ was to act as a civilizing influence. Economic benefits to the empire were secondary (at least in theory). Of course, the moral rationalization for, essentially, expanding one's country and extending power, was important.
A final point to specifically address your question. The social imperative was to expose "uncivilized" populations to the Grand Cultural Tradition of the British which had been handed down since Greek and Roman times. The religious imperative, of course, is to expose "the heathens" to Christianity.