London’s torrential downpours halted, July 7, for the world preem of Warner Bros.’ “Harry Potter and the Deathly Hallows: Part 2,” entirely breaking “Potter” preem law.As the rain clouds dispersed, the waterworks soon began on stage in Trafalgar Square, prior to “Potter’s” final installment, as the pic’s talent publicly mourned the end of their 10-year adventure. Following Emma Watson’s tear-saturated thank you, Rupert Grint threw in a little comedy relief, thanking J.K. Rowling directly: “What you’ve done for ginger people, it’s just, I cannot put that into words, thank you, Jo!” Surrounded by thousands of die-hard fans, Rowling joked about postponing her finite pain, announcing, “You know what? Maybe I’ll just write another one. … That is a joke, but you have no idea. I’ve never come closer than standing here tonight.” Rowling was joined on stage by helmer David Yates, producers David Barron and David Heyman, plus her onscreen Potter trio, including, of course, Daniel Radcliffe. Guests sauntered from the Trafalgar Square red carpet entrance to Leicester Square’s three cinemas, via a red carpet version of the pic’s Diagon Alley, entertained along the way by unusually friendly Death Eaters. Post-screening, guests and talent, including Alan Rickman, Robbie Coltrane and Helena Bonham Carter, journeyed onto Old Billingsgate Market to party by the Thames. The bash continued until it was finally confirmed, in the early hours of the morning, it all ends here.