The 27-year-old singer, who is due to give birth to her second child on December 1, wore a blue Andrea Lieberman gown to the annual event.
She tweeted last night: "Anyone asking where my dress was from at the @BFA tonight, it was @ALCLTD, bag and shoes @LouisVuitton_UK . Congrats Kim Jones you don you x."
The star, hoping to release a new album next year, has revealed that she fainted during a shopping trip at Harrods in Knightsbridge over the weekend.
Allen tweeted: "Just fainted in Harrods. #croquemonsieur needed.
"On the bright side, managed to pick up [a Harry Potter] Professor Snape Wand in Ollivanders box and Slitherin pen for Nephews Xmas present. #luckyboy."
The 'Smile' artist and husband Sam Cooper welcomed daughter Ethel Mary last November, five months after they tied the knot.
See more stars at the British Fashion Awards 2012 below:
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