This is a rare example of an Imperial standard from the Qing dynsaty. This particular flag was among the personal effects of Major Robert Elphinstone Dalrymple Goldingham of the Royal Engineers. As a young officer Goldingham was a member of the British forces sent to the relief of Beijing during the Boxer Rebellion. Family lore, and subsequent research, indicates that it flew over one of the four Imperial navy gunboats captured by British forces during the second relief expedition in August of 1900.  Goldingham was made Major in 1914 and went on to serve in WWI where he won the Distinguished Service Order, the Order of the British Empire, and the Croix de Guerre.
The pattern of the flag, adopted in 1862, exhibits a horned blue dragon rampant on a triangular yellow field beneath a red sun. Yellow symbolized the Manchu royal clan, who ruled China from 1644-1911 as the Qing dynasty. The five-clawed dragon has long been associated with imperial power and benevolence. The color of the dragon, once a dark blue but now faded, was described as "qing" - a homonym for the name of the dynasty.