The function of an MP is to represent his constituents in parliament,not to represent his party in parliament and in his constituency. They may act like that already, but the problem would not be solved by embodying it in the constitution.
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Moon Girl Technologies horrendous beanbag,
Fri 9 Apr 2010, 17:55,
archived)
And PR allows for the electionof multiple parties of almost every viewpoint. You go to the representative you choose. It's no harder to stand as an independent. I think you've succumbed to a few PR myths here.
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John Q Wagonwheel Spent,
Fri 9 Apr 2010, 20:35,
archived)