About RAF use of the P-38, Wing Commander Adrian Warburton used American Owned F-5s (photo reconnaissance Lightnings for the newcomers here) for much of his recce work.
So there must have been a pool of aircraft for RAF use, and it was preferred for some missions.
Didn’t he tend to operate as a bit of a ‘free agent’ in some ways, flying for the USAAF as much as for the RAF?