Yeah I get that guys and I realize that fading out an accent as an adult is a difficult and conscious process but I also thought/think there is some element of involuntary/unconscious change in the way you speak when you live in a local, regarding to the spoken language, environment, especially over such long period of time. This is just a common sense logic I’m applying here that is not in any way pertaining to pronunciation only but can be applied regarding any form of objects - put an object in a different environment over a long period of time and it is bound to be in some way influenced by that environment, more or less.
So what strikes me as odd is not that he has not mastered a perfect English accent over 20 years of living in London, but that he hasn’t been at all influenced in the way he speaks and he sounds exactly the same as he sounded 20 years ago. I would have expected him to have picked up different pronunciation on at least some words, involuntarily. Mark the word different, not perfect or local. It doesn’t have to be good, fluent local pronunciation, just different.