You really are a sharp cookie, and it is a delight to correspond with you.
I'm just here to talk about slavery.
However, although you might indeed be able to SAY these things, IN REAL LIFE people are surely bound to take the ethical/moral issues into account.
Basically i'm a bit old-fashioned in some terms as you may say that i do not appreciate moral issues and you are exactly correct ... In my opinion, the British Empire was right in one way. It reaped its conquest ... So in my opinion, if slavery grants you a victory that can lead to more, than it should be exploited, no matter the moral issues.
And that makes slavery wrong because it is a crime against humanty to enslave people.
YOUR argument - which amounts to: 'let's leave the moral issues on one side, and just say that slavery was good because it was effective' - is akin to Harold Shipman (that doctor who murdered all those old ladies) saying: 'Well let's leave on one side the moral issue that killing people is wrong, and then people will be able to praise the clever ways in which I murdered them and stole their money'.
Equally, your teacher could cut your tongue out and say: 'Well - leaving aside the physical assault - didn't I do well at stopping him talking'.
There's no doubt that you are an extremely clever pupil, but until you take the ethical issues into account in your arguments, you arguments will be shallow.
What makes the slavery debate interesting is the ethical issue: that Britain benefited hugely (basically, the slave trade helped cause the Industrial Revolution) from a wicked practice (and Africa was irrevocably damaged) ... and what are we going to do about it?
And what makes the British Empire debate interesting is the conflict of ethical issues - that the British Empire undeniably brought good things to Africa (railways, hospitals, law etc.) at the cost of depriving the African peoples of their right of self-determination ... so was it a good thing or a bad thing?
It's only when you add the ethical dimension that the thing becomes worth discussing.