If MPs deferred to the wishes of their voters on abortion - after all they are supposed to represent the views of their constitutents - all MPs would vote pro-choice. Personally I see abortion as a human rights matter (ditto gay rights, trans rights, etc) so I think it should be whipped by any party that considers themselves compassionate and just. One in 8 pregnancies worldwide ends in illegal abortion, at a cost of thousands of women’s lives, so it's a big human rights issue too.
I also really don’t understand the idea that anti-choicers are voting based on their religion. No-one I have ever met believes that if I arrive at the pearly gates and say “I don’t believe in God, but I didn’t have an abortion because it was illegal”, they are going to let me in.