Someone Must Remember
 

There's a clip from a mainstream British film where a girl and her boyfriend are chilling on the couch when the girl's sister comes into the room and, for one reason or another, strips off her dress and offers it to the little sister. Another clip from the film that's pure gold is where a roommate challenges that same girl saying she wouldn't walk naked down the street and the girl, again, strips, as she heads down to do just that.
Any help?

That sounds like Olivia Chenery in Legacy, however it sounds like two memories merged together as the scene where she offers up her dress is a different one to where she walks in naked to the couple chilling on the couch.

The dress scene is the one where she walks out naked, (however she isn't seen outside or anything unfortunately)

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