The taking and selling of slaves was common place in Africa during Roman times. The taking and selling was done by one tribe or another to another tribe. The British Empire was just a new place to sell them.
The Irish potato famine happened due to potato blight which was the staple crop of Ireland. Regular famines also occurred in rural areas of the mainland of the UK during Victorian times but are little discussed. Many British landowners did as much as they could to help their tenant farmers in Ireland. Some behaved appallingly and clearly contributed to many Irish people dying - but it was the blight that fundamentally as the problem.
There were many things that the British Empire did that were foul but sometimes the myth making that occurs after the events suits the politics of the current time. Blaming the British Empire for the corruption and repeated failure of Govts throughout Africa and Asia since it collapsed are clear to see.
The British Empire sent millions of its citizens to die in foreign fields and in so doing gave the world a global language, much of the early scientific discoveries we enjoy today, much of the basic medicine, most of the world's sport, great classical literature, art and music. Much of the world's business transactions uses our legal system and our system of Govt has been copied around the world.
It wasn't all bad and frankly if we hadn't done it some other nation, possibly far worse, would have.