While "Illustrious" was being moved in preparation for her acceptance trials on 24 April, the tugboat "Poolgarth" capsized with the loss of three crewmen. The carrier conducted preliminary flying trials in the Firth of Clyde with six Fairey Swordfish torpedo bombers that had been craned aboard earlier. In early June, she loaded the personnel from 806, 815, and 819 Squadrons at Devonport Royal Dockyard; 806 Squadron was equipped with Blackburn Skua dive bombers and Fairey Fulmar fighters, and the latter two squadrons were equipped with Swordfish. She began working up off Plymouth, but the German conquest of France made this too risky, and "Illustrious" sailed for Bermuda later in the month to continue working up. This was complete by 23 July, when she arrived in the Clyde and flew off her aircraft. The ship was docked in Clydeside for a minor refit the following day; she arrived in Scapa Flow on 15 August, and became the flagship of Rear Admiral Lumley Lyster. Her squadrons flew back aboard, and she sailed for the Mediterranean on 22 August with 15 Fulmars and 18 Swordfish aboard.