Premise: The Boulton Paul Balliol and Sea Balliol were monoplane military advanced trainer aircraft built for the Royal Air Force (RAF) and the Royal Navy Fleet Air Arm (FAA) by Boulton Paul Aircraft. Developed in the late 1940s the Balliol was designed to replace the North American Harvard trainer and used the Rolls-Royce Merlin engine, with the Sea Balliol a naval version for deck landing training.
Hypothesis: the Rolls-Royce Merlin engine was the only engine that would fit these planes.