"It's such a pleasure to be back here, out of the darkness," said Rourke.
After her onstage emotional meltdown at the Golden Globes last month, Winslet was a model of composure, thanking her parents, sitting in the audience, "who I will not look at right now, otherwise I will burst into tears."
"Slumdog," Danny Boyle's film about a Mumbai street boy's rise from poverty to game-show triumph, went into the ceremony with 11 nominations and won prizes for best film, best director, original screenplay, music, cinematography, editing and sound.
The low-budget film, shot partly in Hindi, has gone from rank outsider to Academy Awards favorite since it won four trophies at the Golden Globe awards last month and became a box-office hit.
Its makers are still getting used to the change.
"I thought at one stage we were going straight to DVD," said screenwriter Simon Beaufoy.
The film has caused controversy in India, where some have complained it shows the country in an unflattering light, and others have said its title insults the poor.
Boyle dedicated his award partly to the people of Mumbai, where it was shot - and also to people closer to home.
"The wiring in my dad's house blew overnight, and it's just a a big shout-out to everyone who helped him get the extension cable in so he could watch this on television," Boyle said.
The London awards, popularly known as the BAFTAs, have a reputation for predicting who will win at the Academy Awards in Los Angeles. All four of last year's acting prize winners went on to take home Oscars.
Winslet, Rourke, Meryl Streep, Daniel Craig and Penelope Cruz were among the celebs who braved a wintry London drizzle - and hundreds of screaming fans - to walk the red carpet in front of the grand, neoclassical Royal Opera House.
The crowd's biggest cheers were for Brad Pitt and Angelina Jolie - he wearing a mustache, she an elegant, old-Hollywood strapless black-and-yellow Armani dress.
Cruz was named best supporting actress for Woody Allen comedy "Vicky Christina Barcelona."
Ledger's widely anticipated victory was the only win for the "The Dark Knight" from nine nominations. Clint Eastwood's L.A. noir "Changeling" was nominated in eight categories but won none.
"The Curious Case of Benjamin Button," also lost out, taking just three awards - production design, hair and makeup, and visual effects - from 11 nominations.
The 6,000 voting academy members rewarded the quirky and eclectic. Tightrope-walking documentary "Man on Wire" was named best British film, and the prize for best original screenplay went to Ireland's Martin McDonagh for hit man comedy "In Bruges."
Director and Monty Python member Terry Gilliam was given an Academy Fellowship, a lifetime achievement award.
Pinewood and Shepperton studios, mainstays of British filmmaking for decades, were awarded for outstanding British contribution to cinema.
"WALL-E" took the prize for animated feature, French drama "I've Loved You So Long" was named best film not in the English language, and Steve McQueen won the award for best first film for his directorial debut, "Hunger," about Irish hunger striker Bobby Sands.
In the acting categories, Winslet defeated Jolie for the missing-child drama "Changeling," Streep for the moral thriller "Doubt" and Scott Thomas for "I've Loved You So Long."
Rourke beat "Slumdog Millionaire's" Dev Patel, Sean Penn for "Milk," Frank Langella for "Frost/Nixon" and Pitt for "Benjamin Button.
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