> I believe it is 100% the woman's choice
But it isn't. The woman is only legally allowed to make a choice within the 1967 Abortion Act. That means it is not 100% her choice.
[From Wikepedia] The Act made abortion legal in all of Great Britain (but not Northern Ireland) up to 24 weeks' gestation. In 1990, the law was amended by the Human Fertilisation and Embryology Act so that abortion was no longer legal after 24 weeks except in cases where it was necessary to save the life of the woman, there was evidence of extreme fetal abnormality, or there was a grave risk of physical or mental injury to the woman. ... As the UK's healthcare system is completely government-run, abortion remains officially restricted to cases of maternal life, mental health, health, rape, fetal defects, and/or socioeconomic factors.
If the law was changed, the woman's legal choices would be restricted to the new law. Laws are made not by women but by society as a whole, through Parliament. So having a Prime Minister so opposed to abortion would be a threat to the current law, a threat that I don't imagine would lead to much since there would be such significant opposition, but a threat nonetheless.