Thats very true, and its something that continues to this day, sell a US product but not the SAME product we use.
Well in fairness it needs to be remembered that the Brits were buying just about everything that had wings in 1940 without very much regard for performance. The P-38's superchargers were very tempermental, and remained a problem almost to the end of the War. It's entirely likely that there weren't enough superchargers available to equip the British order. Same goes for the Allison engines that rotated in the opposite direction. (Of course that might have been a decision made by the RAF loggy-types to streamline the spares in the pipeline).