Complex sentence: He never graduated, but finished his apprenticeship and gained sufficient qualifications to become a mechanical engineer, and was given a position in the Merchant Navy during World War II, which he never took up owing to illness.
Split into simpler sentences: He never graduated, but finished his apprenticeship and gained sufficient qualifications to become a mechanical engineer. He was given a position in the Merchant Navy during the Second World War, which he never took up due to illness.