Politeness does not change the substance here.
You say you do not hate trans people, but you are also saying there is nothing I could say, no reality you could encounter, and no ordinary human contact that would make any difference to your view that I am not who I say I am. That is not open-mindedness. It is dehumanisation in a more careful tone.
This is how prejudice often protects itself: by keeping its target abstract. It is much easier to hold fixed beliefs about what a whole class of people “really are” when you refuse the kind of ordinary human contact that might trouble those beliefs.
That is a pattern racism knows very well: “I don’t hate them, I just know what they are.” The language changes, the self-image stays flattering, and the refusal to let actual human contact complicate the belief does the rest.
We have a long history of people changing their minds only when they stop seeing others as “them” and start seeing them as human beings. It is much easier to hate an abstraction than a person. Much easier still when that abstraction has been shaped for you by media and places like Mumsnet.
You were free to decline. But refusing even the possibility that reality might trouble your certainty is not caution. It is prejudice insulating itself.
You are willing to discuss this with other cis people, and so is your MP, but not with an actual trans person whose life is shaped by it. That tells its own story.
Whether you like the label or not, that is not compassion. It is fear and hostility dressed up as concern, with the affected people kept safely out of view.