Deliveroo, the British food delivery service, dropped as much as 30 percent in its first minutes of trading on Wednesday, a gloomy public debut for the company that was promoted as a post-Brexit win for London's financial markets. The company had set its initial public offering price at 3.90 pounds a share, valuing Deliveroo at £7.6 billion or $10.4 billion. But it opened at £3.31, 15 percent lower, and kept falling. By the end of the day, shares had recovered only slightly, closing at about £2.87, 26 percent lower.