I have been married for 8 years. My daughter is eight she was born premature and my Australian husband is on the birth certificate. Now we are getting a divorce and he took my child for a holiday in Australia and has not brought her back to the UK where I am.
She is living with him and his parents and when I went to Australia to see her they refuse to let come to the house to see her. They made up stories about me and got a protection order preventing me from going to their house. I had no access to my child and returned to the uk.
If I prove he is not the biological father will he lose the rights to custody and she be returned to me.
His parents are then not the grandparents and she is being held by people who have no right to having her.
I had a brief liaison with someone while dating my husband and when I fell pregnant.
She does not have my blood type or my husbands, we both have dark hair and she has red hair so I think there is a good chance that she is not his biological child.

You need to consult a solicitor immediately. Not sure about UK law, but generally when a child is born to a married couple the husband is presumed to be the father. Unless that paternity is challenged he remains the legal father regardless of biology. There may be a limited window or grounds to challenge paternity. So you need to consult a solicitor who can tell you what the law is.

There is also another factor here. You state you are "getting a divorce". Were court papers filed in reference to the divorce? If so his moving the child out of the jurisdiction of the court will not sit well with the court and you should be able to get a court order ordering his return. Again a solicitor can help with this.

I would not broach the issue of paternity until you learn what the law is.

Papers have not been filed yet as we have to be separated for one year before we can file for divorce.
I have implemented the Hague convention to get her returned to the UK and waiting to see the outcome.