How MPs trade off responsibilities to their parties and their constituents is a deep and long standing one. A decade or so ago I heard a talk by Edwina Currie on how to be a successful MP. Not once, in the hour or so, did she mention her constituents.

So - a problem, right enough. But nothing to do with Brexit.

On the other point, yes, Labour has some interesting problems in the relationship between its voters and the areas which predominately voted to leave. But unless there is an election before March 2019 that will not affect the Brexit terms, which will set the path whoever is in power. So again, I don't think it is really that relevant to post Brexit life. Much more significant is that, for example, Parliament voted against this clause (and apologies for the length):

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New clause 14—Maintaining individual rights and protections—


“(1) When making any agreement under subsection (2), the Secretary of State shall take steps to ensure that UK citizens enjoy standards of rights and protections equivalent to those enjoyed by citizens of the EU under EU law.


(2) This section applies to—


(a) any agreement between the United Kingdom and the EU which prepares for, or implements, the UK’s withdrawal from the EU;


(b) any international trade agreement—?


(i) between the UK and the EU, or


(ii) between the UK and another signatory which seeks to replicate in full or in part the provisions of an international trade agreement between the EU and the other signatory.


(3) In relation to any agreement under subsection (2), the Secretary of State will maintain the highest standards of transparency.”


This new clause creates a duty for the Government to ensure that individual rights and protections are maintained to a level equivalent to (although not necessarily the same as) those in the EU when making agreements with the EU or international trade agreements.
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So Parliament voted 'NO' to maintaining equivalent rights for its citizens. And, relating to the first point, there were perhaps 150 MPs in the chamber to listen to the debate, but - I haven't looked up the exact figures - they almost all voted, based on their party's lines. I doubt if many constituents were writing the MPs demanding removal of guaranteed rights.