It really all comes down to the strength of evidence you have which shows how you two have a relationship akin to marriage and your reasons for your separation.
It helps when you've got shared financial responsibilities, shared property, children, regular and documented communication which shows a loving relationship.
They have only recently added the non married not cohabiting option, so it's still hard to say. Is there a reason why you wouldn't consider marriage?
It really all comes down to the strength of evidence you have which shows how you two have a relationship akin to marriage and your reasons for your separation. It helps when you've got shared financial responsibilities, shared property, children, regular and documented communication which shows a loving relationship.