Though muted in colour, the star still dazzled in her shimmering Valentino, accompanied with her sparkling smile and shiny white satin Tom Ford shoes.
The subtle but still glamorous frock was a sleeveless shift style embellished with sequins.
Scroll down to watch the hilarious trailer for Jen's film Horrible Bosses...
Shining in the rain: Jennifer Aniston was wearing a shimmering stone-grey Valentino dress at the somewhat soggy London premiere of her new move Horrible Bosses tonight
Hollywood glamour: The star looked stunning in a beautifully subtle dress with some gorgeous detailing
Smiling through: Jennifer lit up the grey evening with her high-beam smile
The star soon got out of the chilly-for-July weather however to pose for more photos in the reception of the BFI.
Speaking to MailOnline about the role Jennifer said: 'It was a ball, I had a blast I got totally freedom I got to be a crazy buffoon.'
Speaking about if she found the sex scenes uncomfortable Jennifer said: 'Not with those guys.'
And Charlie Day who plays Jennifer's character's dental assistant told MailOnline: 'It was a a little difficult to make it believable that someone would not fancy Jennifer Aniston but I had a lot of fun with it.'
He added: 'Our director was pretty willing to let us try some things.'
Joining her were her co-star Jason Sudeikis, co-stars Jason Bateman and his wife Amanda Anka, co-star Charlie Day and wife Mary Elizabeth Ellis, as well Radio 1 presenter Edith Bowman and T4 presenter Jameela Jamil.
Hand in hand: Later on Jennifer changed into an LBD for the after party at Shoreditch House in London and left with boyfriend Justin Theroux
Also grabbing the chance to rub shoulders with Jen on the blue carpet were actress Amanda Peet, TV presenter Zara Martin, as well as Made In Chelsea stars Millie Mackintosh, Francis Boulle, Gabriella Ellis, Hugo Taylor and Frederik Ferrier.
In the movie, Jennifer takes on the role of a sex-starved dentist and earlier in the day at a photocall ahead of the premiere, opted for something a little racy in keeping with her character.
The 42-year-old actress wore a simple yet seductive black leather dress.
Top billing: Jennifer back in the warm of the BFI with her co-star Jason Sudeikis
All friends together: Jennifer's co-stars Jason Bateman (L) with his wife Amanda Anka, and Charlie Day with his wife Mary Elizabeth Ellis
Ready for her close up: The star adjusted her famous hair after getting out of the rainy blustery British weather
The star, who is best known for playing Rachel Green in Friends, accessorised with a pair of strappy sandals and two bracelets on her left wrist.
Jennifer showed off an inking, seen on the inside of her right foot, which is a tribute to her dog Norman, who passed away in May.
Jennifer joined co-stars Jason Sudeikis, Charlie Day, Jason Bateman and Kevin Spacey who star in the comedy.
There are 13 years between these two photos. What IS your secret Miss Aniston?
Way back in 1998, Jennifer Aniston had a fresh face, honey-coloured bob and sparkling smile. Thirteen years later she still does.
In the two decades since she first appeared in Friends as Rachel Green, the 42-year-old actress has hardly changed. Some might suggest her devotion to yoga workouts could be paying off - or her careful diet.
How does she do it? Miss Aniston (left) at 42 and (right) as Rachel in her Friends days during her 20s
Then again, as she posed for photographers in London yesterday, it seemed that Miss Aniston is simply enjoying a little fortysomething confidence.
'I absolutely get more comfortable in my body and in who I am as I get older,' she has said.
Broadcast beauties: Jameela Jamil (L) and Edith Bowman (R) had both donned their glad rags to rub shoulders with Jennifer Aniston on the blue carpet
Speaking about her role as Dr Julia Harris, who blackmails her male assistant, she told The Sun: 'It was great fun and very liberating to throw myself into this character.'
'You feel totally uninhibited and get to explore your weirdest and kinkiest psychological quirks and just exaggerate all the kinds of flirtatious ideas and fantasies you've ever had.
'You get to enjoy being fearless when you play a character that is so brazen and domineering.'
Ways and means: Made In Chelsea stars Milli Mackintosh, Hugo Taylor and Frederik Ferrier and friend all managed to get on the guest list for the Horrible Bosses London premiere
South Bank blue: MIC's Francis Boulle (L) and Gabriella Ellis were also at the movie premiere as well as TV presenter Zara Martin (R)
Earlier this month Jennifer was inducted into the Hollywood Walk Of Fame and her new boyfriend Justin Theroux was front row to support his leading lady.
Although she kept him hidden while she walked the red carpet at last Thursday's Horrible Bosses premiere in Los Angeles, Justin was pride of place at the drinks later that night.
After meeting up inside the premiere, the couple spent an hour and a half at the after-party before heading to the Chateau Marmont to meet friends, including Friends co-star Courteney Cox.
An onlooker told People magazine: 'They were super cute. He had his arm around her and kissed her on her cheek. They were whispering in each other's ears.'
Making a statement: The simple but seductive dress showed off the actress's amazing figure and her deep tan
It's also just been revealed that the pair are so serious that Justin has met Jennifer's father.
U.S. magazine Life & Style says Jennifer introduced her father, John Aniston, to her new beau at Delmonico's Steak and Lobster House in Los Angeles earlier this month.
An onlooker told them: 'She was smiling a lot, but you could tell this was a special event for her.'
The magazine said the party enjoyed a long lunch, sharing lobster bisque, Manhattan clam chowder, a crab dish and a four pound lobster washed down with a bottle of red wine.
Comic cast: Jason Sudeikis, Charlie Day, Jason Bateman, Jennifer Aniston and Kevin Spacey all took part in the photocall at the Dorchester Hotel ahead of the premiere
Racy: Aniston in a scene from the film
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