The stars were in town to premiere 3D Disney film A Christmas Carol - a new big screen version of the classic Dickens tale – and also to get shoppers in the mood for Christmas by turning on the capital's festive lights.
Marking the 50th anniversary of the lights on Oxford Street, the display on London's major commercial thoroughfare was turned on by Jim at the same time as those in Regent Street were illuminated by Colin.
Thousands turned up to see the streets transformed into a sea of twinkling bulbs, and also to take part in a world record attempt for the most people singing Christmas carols, led by opera star Andrea Bocelli.
After the switches were flicked, Jim and Colin's new film was unveiled in three cinemas in Leicester Square, and ten more around the country. Organisers hope it will set the record for the largest premiere of a 3D film.
Forty-seven-year-old Jim - who voices several characters in the film, including Ebenezer Scrooge - joked that he wanted it released earlier in the year.
"I told them I wanted it to start earlier but the research showed that closer to Christmas would be better. I wanted this to be a summer release," he said.
The film presentation was a big night out for the X Factor contestants, including John and Edward Grimes, Danyl Johnson and Stacey Solomon.
It was the chance for the remaining eight contestants in Simon Cowell's music reality show to experience their first red carpet event.