"So we are basically back to what I said earlier: the legitimacy according to you lies in the MP's own moral compass."
Yes that's right. That is the basis or our representative democracy. And not "according to me" I have not made it up to irritate you that is how the UK system of Parliamentary democracy works. You voted to retain and promote this system, I would like some liberalising ideas.
That is how the UK system of Parliamentary democracy works. If you don't like it we could join with Europe they have differing systems we could adopt.
This is the very little essence of UK Parliamentary democracy. If you don't like it campaign against it, but it is what it is at the moment.