Complex sentence: The original Yeomen of the Guard (originally archers) chartered in 1485 were all most likely of Briton descent (Welsh, Breton, etc.), established by King Henry VII, himself a Briton who was exiled in Brittany during the Wars of the Roses.
Split into simpler sentences: The original Yeomen of the Guard (originally archers) chartered in 1485 were most likely of Briton descent, including Welshmen and Bretons. They were established by King Henry VII, himself a Briton who was exiled in Brittany during the Wars of the Roses.