I’ve seen the variation with the three cards (known as three card monte) being used all over London. What most people doesn’t notice, and the main reason they fall for it, is that as soon as they approach the crowd around the game, they are already marked by the scammers. The tourist will often see a person (the accomplice) betting all over again and winning, and when he/she wants to try, he/she will often going to win the first or second round. Then it’s always downwards.
I’ve seen people losing 100-200 pounds easily in like 5 minutes, not to mention when they also rob you when you’re distracted by the game. Please beware! If you want to bet or try your luck go to one of the dozens of Casinos spread all over London, never gamble on the streets, you’re simply giving your money away. (Note: if you search on Youtube for “Three Card Monte tutorial” you will see how they do it.)