Summary points:
The loan likely took so long (EDIT: to repay) because they kept refinancing it.
They paid the slaveholders. Not the slaves. This was not reparations to the enslaved for centuries of slavery...
In fact, they actually paid roughly half price for each slave and uh forced the former slaves to work as apprentices for 4 to 6 years to cover the rest of the cost. These apprentices received no compensation.
And then basically the entire history of the abolitionist movement in the UK was whitewashed, written about white abolitionists, many of whom were a ok with other forms of imperialism and colonialism, often directly profiting from it personally.