Who were the most powerful men in england at the time of the outbreak of the 100 years war, 1337?

Answer 1

I think this link will help you find your answer to more information concerning important figures in the 100 years war.

http://www.medieval-life-and-times.info/medieval-history/hundred-years-war.htm

But to answer directly, King Richard II was a pretty prominent figure considering he was the ruler of England, taking his place on the throne in June of 1377.