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I believe that in the Representation of the People Act 1948, the voting system to be used was specified, and its uniformity across all constituencies was imposed. But I guess that could be amended if there was a real desire to do so. It seems unlikely in the foreseeable future, though. Both for practical reasons (as you say) and also because I think there would be a nervousness of a system whereby different MPs might try to claim different degrees of 'legitimacy' depending on how exactly they had been elected.