Locked In is generically a modern Gothic story. British, manor, orphans, illness, love affairs, duplicitous characters, plots within plots, etc. All of that, the movie does well.
Do we need another Gothic mystery of orphaned child becoming entwined with a decaying family and its manor? Probably not. Do the actors move past the 2 dimensional nature of their characters? Decidedly no. Does the story line bring anything new or ingenious to the story? A resounding no.
Part of the issue I see is that the main character is the young orphaned girl and her movement into this family and the machinations of the plot. She's not that interesting of a character. And the narrow focus that the film takes toward her shuts out the other characters from being able to provide much richness to the story.
This harkens to the great movie and story of Rebbeca in many ways. But that film allowed other characters room to play, and the uncertainty of motivations and conniving increased the sense of tension and dread. As a copy or comparison of that, this films decidedly missed the mark. As a display of a Gothic story, it does all right.