What flag would be appropriate?

Either would be historically appropriate. If your project is going to emphasize what was happening in England only, it would make sense to use the English flag (the St. George Cross, the red-and-white one). If instead your project is going to focus equally on events in Scotland, Wales, Ireland, or the broader British Empire (for example, talking about India, British Malaya, Canada, Rhodesia, etc.), then it would make more sense to use the Union Flag (red, white, and blue), which was the flag of the entire empire.