There are over five million British citizens living abroad. Their only recourse for representation in the UK is through their 'constituency' MP. This is not only unfair to constituency residents but citizens abroad who have legitimate concerns and issues which should be considered together.
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It's not fair to any party that the vote and interests of someone in Spain carry the same priority as someone living in Edinburgh. This introduction will remedy a profound democratic deficit and also allow constituency MPs to focus exclusively on the issues of their area.
There are already examples of this system working well - France and Macedonia have representatives for their overseas citizens.