Complex sentence: The BBC introduced the world's first "high-definition" 405-line television service in 1936, and apart from suspending service throughout the Second World War until 1946, was the only television broadcaster in the UK until 1955.
Split into simpler sentences: The BBC introduced the world's first "high-definition" 405-line television service in 1936. It suspended its television service during the Second World War and until 1946, but remained the only television broadcaster in the UK until 1955.