A person's political position may follow from her religious affiliation but it needn't, and to assume it does betrays a prejudice, yes, obviously.
That's logic. But as you suggest, context's everything. The English have a long history of being complete ___s. As a result they've built up an enormous repertoire of racism, sectarianism, bigotry that is never very far from the surface and can be reactivated very easily. As I say you're welcome to inform yourself how all this played out recently in the specific case of RLB, rather than have people explain things to you.
I have worked the Google for you and found this:
https://www.jacobinmag.com/2020/01/rebecca-long-bailey-anti-catholicism-abortion-labour-uk
I'll leave it there before I'm accused of whataboutery, I'm aware this thread is about antisemitism and not English anti-Catholicism. But, you know, all part of the UK's rich cultural tapestry isn't it.