Ashton — who plays Julia Thistlewaite in the upcoming film — walked the red carpet solo, wearing an off-the-shoulder gown in gold chiffon.
Although Hiddleston was not in attendance, Ashton's co-stars Freida Pinto, Sope Dirisu, Theo James and Sianad Gregory all walked the carpet.
Hiddleston, 41, and Ashton, 37, are believed to have been together since 2019, although they keep their relationship quiet.
They were first linked as a couple in 2019 while starring opposite each other in the West End production of Betrayal.
However, they didn't make their red carpet debut until the Tony Awards in September 2021 — although red carpet opportunities were a little light on the ground in the months between.
Fast forward to early 2022, and Ashton sparked engagement rumours at the BAFTAs in London, when she was photographed with a giant engagement ring on her finger.
British presenter AJ Odudu even posted a selfie with the couple and the sparkler to her Instagram story.
Hiddleston previously dated Taylor Swift in 2016, in a widely publicised (if short lived) romance. He's always insisted the relationship was real, describing her as "generous and kind and lovely" after their break up.
Mr Malcolm's List: A Bridgerton-Esque Rom-Com
Bridgerton fans, pay attention: Mr Malcolm's List is a film you want on your radar.
The Regency era rom com follows Julia Thistlewaite (Ashton) as she exacts revenge on Jeremy Malcolm (Sope Dirisu), the most eligible bachelor of the 1818 season.
After he callously dumps her after a night at the opera, she invites her old friend from finishing school, Selina Dalton (Freida Pinto) to pretend to be the perfect wife for Malcolm — before pulling the rug out from under him.
It's not yet clear when Mr Malcolm's List is being released in Australia, but we'll keep you posted.