If they are "ex-pats" I'm not sure why they should be able to vote.
I'm an ex-pat, I haven't lived in the UK for yonks. I don't expect to vote on decisions affecting a country in which I don't live.
First off all, not everyone that lives abroad has done so for years or decades.
On top of that: The outcome has very real consequences for those UK passport holders outside the UK. Not letting the people have a vote, that are probably among the most effected citizens you have, is about as undemocratic as you can get. It is after all about people deciding the faith of their own lives, and some 1.8 Million people have been excluded.
More importantly, how democratic is a system in which the government can handpick the electorate to fit the desired outcome? "Oh, young people rather want Scotland to be a part of the UK, let 16 and 17 years old vote".
People have been excluded from voting because of decisions they made for their private lives, like falling in love with a foreigner or, more devious, actually moves abroad because their, UK, company send them there and could, maybe even had to, do so because their country did join the EU.
They got their right to democratic representation stripped from them without doing anything wrong or breaking any laws. That is a slippery slope. Why not exclude people that have tax debt? Need welfare? Any reason to exclude a citizen from voting is wrong. That can clearly be seen by our philosophical progress, old democracies tend to exclude prisoners from voting, new democracies don´t.
On top of that, voters have been presented with only a binary option, when clearly more options are actually on the table:
1) stay in the EU
2) leave the political structure, but remain in the common market, immigration and all
3) leave the EU and the common market, be truly independent.
They lumped 2 and 3 together, and by that token "forced" themselves into doing something that will in all probability not get a majority vote. You can decide many things with a yes or no vote, but this issue is apparently unsuited for that much condensation.
In the end there will be a 2nd referendum, either because the government sees that they don´t have a clear mandate due to the above and make a new referendum with more options, or they put the negotiated treaty with the EU up for referendum, with the alternative being to apply to EU goodwill to take them back as a full member (no rebate, no special rules). Or something like that.