King looked gorgeous in a green patterned dress which she teamed with a pair of red peep-toe heels, while Flack kept covered up in a black mac and chunky black sandals.
Other guests at the party included James Corden, a fur coat-clad Laura Whitmore, Marina Diamandis of Marina & The Diamonds, Hobbit actor Luke Evans and TV presenter Zoe Hardman.
Meanwhile, Smith, 30, set tongues wagging after he spent the evening chatting away with 20-year-old Angus, Thongs and Perfect Snogging actress Georgia Groome before the pair hopped into a waiting car and left together.
It was reported recently that the actor had rekindled his relationship with on/off model girlfriend Daisy Lowe after the pair were spotted looking close at the Esquire and Tommy Hilfiger party earlier this month.
Yesterday's InStyle bash, in association with Lancome and Avenue 32, saw model Beck Tong take to the decks and French trio We Are Evergreen perform a live set.
See more pictures from the InStyle party here.