AFAIK, the Allison pistons were never fitted to a British aircraft design
Technically correct
Danny, but the P-51 was designed to fill a British contract, the Brits paid for that, the development and purchased some 600 aircraft prior to the US sitting up and taking notice. The Brits also provided the Meredith system for the cooling system, and a Brit aerodynamics' chap sorted out the overheating problems. So, could be said the design was a joint US/Brit, even if the Brit contribution was minor in the scheme of things. Oh, and Allison engine of course. Without the Brits the aircraft would not have existed.