The fact that there is no example of a large, unitary population with PR in Western democracy is your first piece of evidence – though circumstantial of course. In that vein, we can go further and see that France, likewise with no significant devolution, also lacks PR (they tried it once and switched back). Further still and note that PR DOES exist in UK, but only with smaller electorates.
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UK requires modern devolution to be able to have proportional representation
