The question's moot as far as I'm concerned as I rarely go over 20mph. Certainly not in built up areas with loads of hazards, turnings, parked cars, pedestrians crossing and so on which is what the 20mph zones where I live are like.
But in the incredibly unlikely circumstances that I was going over 20mph by a significant amount and capable of maintaining it for some distance, making an overtake worthwhile, and it was safe to overtake, and there happened to be a car going at at or under 20 mph then ... No, I wouldn't overtake it. I'd adjust my speed to that of the car.
But it ain't going to happen.
Drivers have to put up with the fact that I'm slower than them so in the incredibly unlikely event that the tables were turned, I'd return the favour.