I'm guessing they're mostly referring to the history of colonialism by imperial Britain and King Leopold of Belgium. I'm not sure anything was on the scale of the extent of the Nazi regime's worst actions (though Leopold wasn't far off), but there were certainly many atrocities.
Oh my. Well just off the top of my head. The UK has committed widespread systematic war crimes/crimes against humanity for generations in Ireland (+ is responsible for the deaths of the famine), Kenya, India, South Africa and localised war crimes in Iraq, Afghanistan and many Asian countries. Bombing of Dresden also. To name but a few. Then you can probably argue forcing and dumping opium on China is another one.
