It happened before any of us were born, if I or any member of my family continued to be involved in slavery then I would have something to apologise for.
But several hundred years ago we, the British outlawed slavery, (even though it had brought some of us riches).
Slavery was used at the time and people of that time were ignorant and considered it normal practice, if we, as 'enlightened' people still used it as normal practice then we would have something to apologise for, but we don't use slavery.
We of this day and age should not be taken to task for what was done before we were born,
That, couldn't be influenced by any of us.
Life will give us our own reasons to feel guilty.