"...this mother has forfeited the right to bring up her own children."

Talking in terms of parents' rights is beside the point - the rights of the child are what is paramount in British law.

If the interests of a child are seen to require that a child is taken away from the parents, any parental right to bring up the child just doesn't enter into it.

And conversely, if the interest of the child is seen as requiring that she should stay with a parent, there can be no question of that being prevented because that parent is seen as having "forfeited the right to bring up her own children".