I think your view of history might be a little over simplified. The american revolution wasn't like hollywood, it was essentially one group of mainly british and irish people fighting another group of british and irish people. The colonists were fighting for their rights as englishmen, and many people in the british isles supported them in their struggle, at least until they invaded canada.
Parts of the empire that were settled by colonists (New Zealand, Austrailia, Canada) were gradually encouraged to become more and more independent by britain. Developed areas gradually conquered by the british such as india and egypt (or more accurately, by indians with a few british and irish commanders) were exploited and post war nationalist movements got them to leave. Undeveloped unsettled areas such as the afican colonies britain was eager to get rid of in the 1960's as they spent far more money developing infrastructure than they ever extracted. Most of the colonial wars they fought they won (Malaysia, Kenya) but they didn't want to hold on to the colony, they usually just wanted to stop it being communist after they left. Ireland was partitioned after a successful war of indenpendence after WWI as it was effectively half settled (North Ireland) and half exploited.