Nope. Does not exist. There is no such thing. You are either married/in a civil partnership or you are not. And the law is very different in each. I'm just going to leave this here (click to enlarge): …
marriage
Married, sort of married, not married. Its all the same, right?
Well, no. Not really. In fact, not at all. To put it simply, you are either married (or in a civil partnership) or you are not. And that is it. There is no such thing as common law husband/wife. Nor are you 'assumed' married or treated like married people if you have lived together for 5 years, …