Through history, the British Royal Family has lost any number of men to active combat, but it’s a number that has dwindled considerably in more recent centuries. The last king to die in battle was Richard III in 1485; the last king to actively participate in one was George II in 1743. Since then, the trend has been to preserve monarchs and from there direct heirs to the throne. Younger sons have a bit more wiggle room, most recently evidenced by the top secret deployment of Prince Harry last decade.
The most recent war casualty of a senior British royal was Prince George, Duke of Kent, fourth son of King George V and Queen Mary, during World War II. Aged only 39, George died from an airplane crash near Caithness, Scotland on August 25, 1942 during non-operational duties.