Slavery falls under "nasty things the British Empire did but we don't want to admit to as that doesn't fit with our notion of being British means being the best morally and in practice".
Also see Boer War, British in Indian, transportation of criminals, any other British colony.
There are huge amounts of ugliness.
Our British ideology is all about how Empire made these places better than they were originally, civilising them and improving things for 'the uneducated natives' or 'giving the undeserving poor an opportunity'. Which is quite some bullshit.
What the subjects and possessions of the British Empire had to endure in terms of things which were against just about every human right law today is horrendous.
It is all part of why so many British don't value human rights as much as they should. They are superior to them and think they have no need for them.
Even those who think they support human rights often have surprisingly poor knowledge about human rights were created. And the why is fundamental to their continued existence. We are seeing attempts to ape human rights in new areas which don't fit with the principles of why human rights exist : to protect the most vulnerable and the voiceless, those who are seen as unworthy and worthless in society, by various groups who do have voice and political influence.
I have in my family the story of my great great great grandfather.
He was a solider in the army in Indian around 1856. During the Indian rebellion there were many massacres and atrocities that were carried out by both sides. The Indians were punished heavily for it, but few British soldiers were held to account for their barbarity. My great great great grandfather was an exception. He was court martialed after murdering an Indian just outside Cawnpore just after the siege and subsequent massacre there and was found guilty.
He was sent to Portsmouth and put in prison there. At the time many prisoners were held in prison ships as they could no longer transport to America or Australia and we didn't have enough prisons on shore (a third of prisoners died in the conditions on board. They were often there for petty theft they had committed in order to survive), but he didn't stay on board one long.
He was given prefential treatment by the governor who took him on land and put to work for him personally in his quarters decorating.
The governor viewed him as a miscarriage of justice and supported an appeal for his pardon by the king. He won, and was released. He later married and had a daughter... Who I am descended from.
I can not find details of the murder he committed. If they still exist they will be in regimental records, which are difficult to access. I guess the circumstances must have been pretty brutal for him to be court martialed when so many weren't. His prison records do record the name of the Indian he killed, which is something at least. I will follow it up the story one day.
The story that was handed down within the family was he had been a hero during the Crimean War. The reality we found was altogether different.
Justice of the past was far from equal. It remains the case. And the stories we tell ourselves are often very different to what happened.
I don't think you need to know everything about slavery nor the British in Indian etc. Just a few human stories suffice to get the general idea.
(Before you think everyone in my family is a murderer, DH's family are responsible to the other one!)