I always got the impression that those videos were dubbed. His English is just too perfect.
There are plenty of videos where Lutoslawski is speaking extemporaneously and the audio perfectly matches his mouth, so no, obviously not dubbed.
It is not particularly astonishing that Lutoslawski could learn English so well. The Eastern European elites who came of age in the
entre deux guerres or before often learned French or German to the same high standard, and Lutoslawski came from an aristocratic family. My question was only about how he managed to immerse himself in an English environment for a time, because this wasn’t covered in any of the biographies which I have seen.