It's certainly apocryphal and you have the British version, probably. In the US it is usually Joe Kennedy and a shoeshine boy, and also didn't likely happen. These stories are useful parables, and they serve the purpose of explaining why the smart money didn't get cleaned out when the rubes did.

Still, if a 10 year old had started investing 10% in the market in 1920 and stuck through it during the depression, even with no income coming in at the time, they would have done handsomely through the recovery and into old age. In fact, a middle aged person who had been investing until 1929 would have not been fully cleaned out, and that money would have recovered its value by 1943. Margin was what killed fortunes in the day, so the lesson to learn is to avoid margin for your investment portfolio. (Speculation is a different story).