Complex sentence: The G3 had been favioured ahead of the N3 because of the expected slow supply of the guns; only one British company could provide the 18-inch guns whereas there were four that could build 16-inch guns readily.
Split into simpler sentences: The G3 had been favoured ahead of the N3 because of the expected slow supply of 18 inch guns for the N3s. Only one British company could provide the 18-inch guns whereas four could build 16-inch guns readily.