Complex sentence: Other UK World War II munitions factories were built and owned by Imperial Chemical Industies (ICI); these ICI munitions factories were not considered part of the ROF organisation and they were not called ROFs.
Split into simpler sentences: Other UK World War II munitions factories were built and owned by Imperial Chemical Industries (ICI). These ICI owned munitions factories were not considered part of the ROF Organisation and they were not called ROFs.