Complex sentence: British cavalry were excellently mounted and were reckoned superior to French cavalry if squadrons clashed, but because brigades and even regiments were rarely exercised in battlefield manoeuvres and tactics, they were inferior in larger numbers.
Split into simpler sentences: British cavalry were excellently mounted and were reckoned superior to French cavalry if squadrons clashed. But because brigades and even regiments were rarely exercised in battlefield manoeuvres and tactics, they were inferior in larger numbers.