There are times when the job of a party MP is to toe the party line. If you stand on a manifesto, that's what you are voted in to push through. However, even on manifesto items, after all the legal eagles have had their way with a bill and it's been through the committees and back and forth to the Lords there are some topics where the individual MP is forced to address questions like "is this right or wrong?". This "acid test" is what we criticised Hitler's minions for not applying. The job of our MPs is not "just to follow orders". I have no problems with their holding a public meeting to discuss a topic, or leafletting their constituencies to elicit other people's views. But even after all that has been done, I still respect the right of the individual to say "not on my watch".
It's a bit like being on a jury - sometimes you have to be the awkward sod who says "Not Guilty" when everyone else just wants to get back to work.