That explains it...
It seems that nearly every lesbian love story ends tragically: Lost & Delirious, Boys Don'tr Cry, Willow and Tara on Buffy The Vampire Slayer. Its a strong enough pattern that I had misgivings going into the end of this movie. Only to be surprised when the heroin leaves Tamsin in the creek soaking wet and alive (and obviosuly her living as well). Its almost to scratch one's head. and then I come on this board and the first post I read says that they weren't really Lesbians. Huh? they were just rebelling against men? okay. that, I suppose, would explain the difference.
Even if the main character was actually gay and would go on to have lesbian relationships it could be argued that Tamsin(Emily Blunt) was probably faking it (part of the big role play) and feeding off the heroin's attraction to her. I can deduce then that if they shared true feelings for each other, one of them at the end, would have commited suicide or been murdered by the other, or get murdered by a gang of bigotted males (perhapos the Paddy Considine character would be involved if the latter).
But wait a minute you say, what about Kissing Jessica Stein? Nobody gets killed at the end of that one. Again, not really a lesbian in that movie. Or Imagine Me & You with Lena Headey and Piper Perabo? Two definite lesbians in that one and nobody died there! If you go to the IMDb trivia entry for Imagine Me & You you'll see it was originally written for a heterosexual romanctic couple. It was a fluke.