The United States, Canada, Australia, and New Zealand obviously turned out quite well largely modeling British systems of common law, trade, treaty, military organization, and government.
and
The African countries were much better under British Rule than non-British rule.
I'll ignore the fact that you're trying to mix together countries that have been independent for centuries with other ones which only acquired it roughly 50 years ago.
What I don't understand is your circular reasoning. If the rule under the British empire was that great, shouldn't the US and Canada be in a much worse state right now compared to that golden age? This seems to contradict your thesis.
And lastly, you seem to forget that most of the conflicts in Africa are due to the artificial borders imposed by colonialism. Some tribes like the Tutsi are spread across Rwanda, Burundi and Eastern Democratic of the Congo. Bring in the fact that most of these countries were governed by dictators largely supported by the former rulers and you'll see a cleaner picture of the situation.