If the non biological parent is not on the birth certificate they have no parental rights over that child.
True but they can aquire legal parental rights by following the correct process.
From the governement website.
Who has parental responsibility -
A mother automatically has parental responsibility for her child from birth.
A father usually has parental responsibility if he’s either:
- married to the child’s mother
- listed on the birth certificate (after a certain date, depending on which part of the UK the child was born in)
Link
https://www.gov.uk/parental-rights-responsibilities/who-has-parental-responsibility
Apply for parental responsibility -
If you’re not the mother, you can apply to court to get parental responsibility.
You need to be connected to the child, for example as their father, step-parent or 2nd female parent.
More than 2 people can have parental responsibility for the same child.
Link https://www.gov.uk/parental-rights-responsibilities/apply-for-parental-responsibility
According to that assuming one of the lesbians is the mother she gets parental rights automatically. The second can apply through the courts, it even mentions "2nd female parent", to get parental rights.
Why would anyone want to take away a lesbian's right to be the legal mother of a child they have raised.
No one is taking away anyone's rights. You are imagining rights that don't exist. Lets say two lesbians have a kid together, one of them is the mother. The mother gets rights automatically. Both lesbians then apply jointly to the courts for the second to get legal parental rights, they get it, imagined catastrophe averted. During that time they both behave like parents anyway because they are a couple. It's just some paperwork.
How anyone is taking away anyone's rights after they have raised a child you'll have to explain. Perhaps if they separate the mother's rights take primacy, but like any separation with kids it could end up in the courts. The mother lesbian or otherwise retains her parental rights because she's the mother.