Ashley Jackson's Very Short Introduction to the British Empire
This book sets out to answer questions without bias since this subject attracts strong views on both sides. One point he seems to miss early on is just why the British sought to impose their ideas on the peoples they governed.
Let us suppose that I have a better way of thinking that you have. Common sense would suggest that I keep my secret to superiority rather than share it. The British thought their way was better but then went on to try to share it with most of the world's population.
I suggest that this was central to the 19th century view that people could be improved and so be more likely to be among the saved of God. It is a mindset we can no longer readily grasp but I think it unlocks some of 19th century thought