A reason in favour of expats voting is that they should have a say on things that affect their citizenship. You can definitely argue how far that argument hold

A reason in favour of expats voting is that they should have a say on things that affect their citizenship. You can definitely argue how far that argument holds sway - after all, any government could change its citizenship rules for expats. But Brexit clearly was a different case than regular elections. The government was embarking upon a sea change for expats - changing the rules of the game while playing.