What was it? c 200 British and Imperial General Officers (including Brigadier-Generals) killed on operations in World War I...and that's before we look at field rank and junior officers.
Worth a watch:
Victorious Donkeys? British Generals and Generalship of the First World War Reconsidered | University of Oxford Podcasts - Audio and Video Lectures
The Professor of War Studies at Wolverhampton University, a leading British military historian of the First World War, explodes some myths about British generalship and the performance of the British Army.