I saw this movie a few years ago, it was an English movie (in color ofc) where some killer came into this woman’s house and she was in the bathroom (it may have actually been a hotel room) and she heard them so she scrambled for a weapon, so you smash the mirror herself and used a shard as a weapon. That’s all I remember and it’s driving me nuts. I think the killer was also a female.
Your description sounds very similar to the French horror film “Inside.” You mentioned it was English, so not sure it’s the right one but thought I’d call it out just in case.
That was not it unfortunately.
possibly Single White Female
or The Hand That Rocks the Cradle