Complex sentence: Until the advent of air transport and the creation of the Channel Tunnel, marine transport was the only way of reaching the rest of Europe from England and for this reason, maritime trade and naval power have always had great importance.
Split into simpler sentences: Until the advent of air transport and the creation of the Channel Tunnel, marine transport was the only way of reaching the British Isles. For this reason, maritime trade and naval power have always had great importance.