Complex sentence: Racal Electronics plc was once the third-largest, British electronics firm, before being purchased by Thomson-CSF (now Thales Group) in 2000, as a way for the French firm to gain access to the lucrative UK defence and armaments market.
Split into simpler sentences: Racal Electronics plc was once the third-largest, British electronics firm. It was listed on the London Stock Exchange and was once a constituent of the FTSE 100 Index before being purchased by Thomson-CSF (now Thales Group) in 2000 thereby giving the French firm access to the lucrative UK defence and armaments market.