The Rolls-Royce Spey powered the British F-4K and F-4M Phantom II variants.
The Spey was a turbofan engine developed by Rolls-Royce. The Royal Navy’s F-4K Phantom, also known as the Phantom FG.1, and the Royal Air Force’s F-4M Phantom, or Phantom FGR.2, used British Spey engines instead of the General Electric J79 engines fitted to many American-built Phantoms.
The Spey’s greater airflow helped the British Phantom variants operate from the smaller decks of the Royal Navy’s aircraft carriers. However, installing it required substantial changes to the Phantom’s airframe, including modifications to the rear fuselage and intakes.
This fact is easy to mix up because the Phantom family used several engine arrangements across different operators. The American F-4’s standard powerplant was the J79, while the British versions were the Rolls-Royce-powered exceptions.