I taught history for eight years in public schools and was interested in this myself. A book that I really think you should check out is History Lessons by Dana Lindaman. She has chapters where she looks at the same historical event from multiple perspectives - the American Revolution from Canada, France, Great Britain, the Caribbean, and Germany. It does a good job of looking at the same events that we're used to, but also bringing up collateral issues that weren't important enough to be included in American textbooks that had a very large impact on other countries. Her section on World War II in the Pacific includes information from the Japanese and Filipino perspective.
This only covers World War I, but Intimate Voices from the First World War by Svetlana Palmer does a fantastic job of bringing in different perspectives from people in World War I. The accounts from Piete Kuhr, a young girl in Germany, are particularly haunting to me because it was showing how everyone back at home was suffering, too.
Not too sure about the education system in the UK, but they have what's called the GCSEs for students between ages 11-16. Might be about the level you're looking for. Pearson has some books on particular subjects for the GCSE. Another company, CGP, has some books, too. You may contact those companies or others, explain your situation, and ask for something like an instructor's copy, teacher's copy, or examination copy of a textbook. I remember being able to get a few free textbooks using that procedure when I was still in the classroom. Good luck!