Step-Parents and Parental Responsibility

Was Just wondering if anyone can clear this up for me...

Me and My Partner have a 6yo little girl, nothing unusual there though, the thing is, there is a 50/50 chance the little'un is not mine...

She got out of her last relationship and then Me and my Partner were on and off for years, and I understood back then there was a 50/50 chance of me being daddy.

I promised to stand by her whatever with one condition, my life was a mess at the time and I had sort myself out, which meant I stayed away for nearly 5 years, which was a mutual agreement between and my partner, I was determined to stick to my end of the bargain and did, and Me and the little'un met back in 2015, I am listed on the birth certificate as Dad despite that 50/50 shot as I wanted to do whatever it took to make sure she is brought in a stable home.

Her last partner wanted nothing to do with them and even went as far as offering to sell the little'un, when my partner said no, he walked off wishing death on them both nearly 7 years ago.

Where do I stand legally on this, for example, if something happens to my Partner after wr get married, because as far as the little'un knows, I am Dad, I could not be there for a while for understandable reasons and I listed myself as Father knowing of the 50/50 shot, have I done wrong in doing so and can my position be challenged, because I will fight anybody who tries to take my kid away from Me and would instantly go down the adoption route if that was the case. She knows me as dad and I will be damned if anyone is going to let her think different.

I came into this wanting one thing only...A stable family

Unless the mom is threatening to leave you, I cannot see why, with your name listed on the birth certificate and marriage mentioned, fear of losing your child is noted?
What about a paternity test to clear things up?

You are going to have to talk to a family lawyer about what the legal situation. In the US you would be the legal father as you are on the birth certificate. However, I don't know the law for England. My guess is you have good standing if something happened. A lawyer would be able to give you better advice. If you are thinking about death (and not that mom would leave your relationship with her) then if you are concerned talk to a lawyer and she could also back up you having the kid by putting it in her will or other legal documentation that you are to have custody of the child. If you are concerned about her leaving your relationship then it is you talking to a lawyer to find out what your legal standing is. Good luck.

The mother will have to have the biological father's rights terminated on grounds of abandonment/disinterest before you can legally adopt her. But I wonder, if you weren't married, how did you get to be on the birth certificate? As far as I know, any unmarried parents have to establish paternity before the father can be put on there.