Thing is our linguistic brain evolution almost totally stops after the age of 4 or 5. Or it becomes incredibly less efficient in deciphering and registering new linguistic sounds.
So I can understand why your british guy can't comprehend even after being explained. This also explains why it's so hard to learn languages that are too different from your natural one regarding sounds, rythm and intonations.

Regarding the McGurk effect, it worked on me. But I might be even more affected than anyone else, because since English isn't my natural language, I need to read on the lips even more than you do even in that video since it's presented in English. But even then, the sounds are so simple that I'm ashamed it worked that well. Closing my eyes proved how stupidly efficient this is...