LONDON—Steve Scotland had better reason than most for thinking he knew London like the back of his hand. Not only was he a native Londoner, born and bred, but he'd spent years working as a chauffeur in the city, driving his passengers wherever they wanted to go, finding the shortcuts, negotiating the city's traffic-clogged streets swiftly, accurately, and with a minimum of fuss.

So he quietly fancied his chances of passing "The Knowledge" test—the demanding test of London's back streets and landmarks that confronts anyone who wishes to join the elite ranks of London's cabdrivers.

"It was something I always wanted to do," Scotland says.

In pursuit of this dream he went to the Public Carriage Office, which regulates taxis in