Being founded on slavery is different to inventing it. I'm sure students understand that. The cotton and sugar industries relied on slaves and other countries were involved in that trade, including the UK. Of course it was considered normal by those in control then. We can't deny it happened and that it was terrible. This is what happens when some people are considered inferior and extreme patriotism can edge that way. That can be how wars are justified.

We can also talk about how the British empire was built on the exploitation of other cultures. So were other empires. We would hope to be better now, but bad things still happen.

I believe we can love our countries whilst acknowledging their history and respecting other cultures. Students are very capable of thinking for themselves. They may be less prone to believe propaganda than some older folk. You just cannot generalise about any set of people.