When the troop ship taking the platoon home is bombed and sunk in Dunkirk harbor, the scene of the ship sinking is taken from The Cruel Sea (1953), which depicts the sinking of H.M.S. "Compass Rose".
The term "U-S" means 'un-serviceable' and is a British Army term.
The last words uttered on-screen by the dying soldier played by Meredith Edwards are in the Welsh language, although they are barely audible. The lines were written in English originally, but Edwards argued that a Welshman who knew he was dying would want his last words to be in his native tongue. The director agreed, and Edwards spoke in Welsh.
The command center used by Vice Admiral Ramsey (Nicholas Hannen) was the actual bunker used during the operation. It's part of a large underground complex beneath Dover Castle. It's open to the public. The balcony in the cliffs is visible from the ferry terminal below.
The beach sequences were shot at Camber Sands on the south coast of England, while the town of Dunkirk was re-created in the nearby English town of Rye.