My FiL sailed on the Atlantic convoys during WW2. He rarely spoke about it, but he survived the storms and U-boat attacks.
In 1944 he was on his ship in Portsmouth when two official looking gentlemen came on board and told him and his mate to go with them. They were only told that they required experienced people for a special task.
Next day was D-day and my FiL found himself on a ship ferrying munitions and supplies to back-up the invasion.
I lived for many years near Portsmouth and visited many of the WW1 & WW2 sites both in the south of England and in France & Belgium.