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I imagine that a Briton army would have a heavy focus on archers, light cavalry, slingers and spearsmen in any eventually. The Angles, Jutes and Saxons would provide heavy infantry and ships, of course. As regarding a strong cavalry component... You might want to perhaps include something of a Roman or Sarmatian nature to flesh it out. Or perhaps you could take a step beyond, and merge the archers, slingers and cavalry into missile cavalry?