What people forget is that the norms of the past are not the same as the norms of today. At one time, slavery was normal across vast swathes of the planet. That it isn't today, is in part due to a moral revolution that took place in Britain and resulted in a policy change by the British empire - despite its own commercial interests - to abolish slavery and the trade in slaves, even while nearly all other countries in the world at the time were totally fine with it.
I think you may not be aware of the graylevels within that statement. It is true that the Slavery Abolition Act 1833 abolished the African slave trade. But you need to know the details to realize why the powers that be allowed that to happen. Also, it is worth mentioning that while the same 1833 act made an explicit exception for territories possessed by the East India company. Further, slavery was replaced by indentured labor. This is why you find large populations of non-African slave descendants in many countries. If you've ever vacationed in Trinidad, you'll notice lots of Asians (Indian, etc) people. They got there because their ancestors were brought as indentured labor to replace African slaves. In many cases, as is still occurring today in places like Qatar/UAE, indentured labor is just a #define for slavery.
"while nearly all other countries in the world at the time were totally fine with it". I guess this depends on how you define "country", but lets take it to mean the majority of a population. I'll give you that countries benefiting from the slave trade were clearly fine with it. I sincerely doubt the populations being enslaved were though. (their opinions count right?)
You can cast this in cynical terms as self interest, but you can do that with any action by any state in history - Britain is hardly unique there.
Its basic economics. Had the ruling elite deemed it necessary I'm sure Punch could have been commissioned to print all manner of xenophobic cartoons about subhuman slaves and their barbarism in order to justify the practice further.
This isn't to diminish the efforts of the anti-slavery movement or the individuals involved.
*Edit: Also, yes, this is not unique to any country at all. I'm sure there are a myriad of examples of slavery within any of the countries that Britain colonised throughout history.
Slaves made empires (true empires) possible because captive people are a net positive for the conquerors. You could conquer your neighbors and... sell them to pay for the war, which isn't the case today. Outside Syria, I guess. When the Roman Empire was at its peak citizens could pay their taxes with only two days of work per year.
If you look at the contemporaries of the British Empire they do not stand out as exceptional either good or bad morally. Meanwhile, if you compare the Nazis to their contemporaries they do however stand out (though not nearly as some people might think).
To me it seems rather cruel and unfair to label someone or something as "immoral" if they are acting within the limits of accepted and expected behaviour in their environment.
I find it ethically very dubious. Certainly many people committing atrocities have been "going along with the flow" of their own local environments.
Bartolomé de las Casas, Martin Luther King Jr., Gandhi etc, all of these people are looked back upon with great respect because of their morality - but the truth is that none of them would seem like morally great people today. They stand out because they pushed further on certain issues compared to their contemporaries, not because their thoughts and actions would stand out compared to ours (other than that they would seem very backwards and immoral concerning a lot of things).
Thereby making them immoral by their own standards. You cannot export your sense of locality to somewhere else, i dont think thats how it works.
Clearly you feel that anybody of this opinion is in error. Does everyone who posts an article about the nastiness of the (historical) British Empire feel the same way? There are quite a few come up on Hacker News. To me it doesn't seem like England is perceived as lovable at all, in fact I can't imagine where you got the idea.
You mention the Nazis, but I don't think I've ever seen a post about those guys. It's almost as if one needn't hold modern-day Germans accountable for the crimes of their forebears? What a strange idea.
It is difficult though to distinguish neglect from intent sometimes, with "The vast majority of Boers remaining in the local camps were women and children. Over 26,000 women and children were to perish in these concentration camps. [...] The inadequate shelter, poor diet, bad hygiene and overcrowding led to malnutrition and endemic contagious diseases [...]"