Exactly Rob, to to cast your vote ,you have to vote for a candidate of your preferred political flavour ,in your constituency where you reside.Those who vote by post have the ballot papers sent out to them from your constituency.You have to be on the electoral roll.
So how on earth can ex-pats vote from abroad,unless they are falsely claiming to live in a constituency in the U.K. I have a feeling some of them are getting a proxy here in the U.K.(who declares that they are residing at the same address) to do it for them .Can of worms indeed.