There is an offence of Child Destruction in UK which applies to the unborn. The man charged with the recent murder of Nikkita Grender was also charged with this because he also killed her unborn child. It applies only to acts which are intended to cause death (so doesn't cover eg smoking, drinking and drug use in pregnancy as they would typically be carried out for other reasons).
I think it can be applied to abortions carried out outside the provisions of the abortion act, and could apply here to the one off ingestion of rat poison with the intent of destroying life (hers and the child). But it's complicated as suicide isn't an offence, and because the mother in this case has mental health issues. So I doubt it would result in a murder charge in UK.
But this woman does appear to need help, so a charge (with a diminished responsibility mitigation) might be a means to secure for her the help she needs.