EPSOM, UK—"I'm not really a person to get over-excited about things," said Adam Kirby as he stooped over the podium after winning his first Classic, the most important one of them all. For the tall, gaunt jockey, so admired by his peers but with nothing like the rock star profile of Frankie Dettori, even riding at nine stone is a struggle. Slow to face the press for the post-race conference, he admitted he'd taken his time in the weighing-room so he could have a bottle of juice while gathering his...