The actress chose a low-key look as she covered up with a black hoody underneath her black leather jacket, reported E!Online.
Actor Rob Lowe spotted her and stopped to snap a photo with the mother and child duo.
"My flight to London was filled with many of my fellows. Here's the fantastic Rosamund Pike with her tiny treasure," he captioned the picture.
This child is the actress' second son with partner Robie Uniacke. The pair gave birth at home on December 2 but they are yet to share any pictures of their little one.