Sure enough, the young man attracted no attention.
The photo of the "German officer" posing in Central London ran in a lot of papers, both in the UK and America. (For instance, the Lewiston Morning Tribune - Apr 29, 1941).
But I think the experiment would have been a lot more interesting if he had walked through London wearing a uniform with all the Nazi insignia still on it. That would probably have elicited a different reaction.