The brigade ended its operations in North Africa in mid-April 1943, and Flavell left for England in June to take up a new command. He was replaced as commander of 1st Parachute Brigade by Brigadier Gerald W. Lathbury. He became commander, Airborne Establishments, which was tasked with promoting closer cooperation with the Royal Air Force as well as providing reinforcements for the 1st Airborne Division and aiding in the creation of the Polish 1st Independent Parachute Brigade. Then on 12 June 1944, he was given command of the 6th Airlanding Brigade, part of the 6th Airborne Division, commanded by Major-General Richard N. Gale. On the night of 5 June the division conducted Operation Tonga, the British airborne portion of the Allied invasion of Normandy. As an airlanding brigade equipped with gliders, the brigade was the last unit belonging to the division to arrive, landing at 21:00 on 6 June in landing zone 'W' after it had been secured by the rest of the division. Flavell continued to command the 6th Airlanding Brigade throughout the period that 6th Airborne Division fought in Normandy, as well as when the division fought in the Ardennes forest during the German offensive there in December 1944. He was involved with "Operation Market Garden" at Arnhem where his son, James, also fought and another son was involved towing the gliders for troop landings. He was then appointed as Deputy Chief of Staff HQ in First Allied Airborne Army, and subsequently made an Officer of the American Legion of Merit.