This isn't quite right ...

[Kraut]

A non UK national shouldn't be allowed to vote for the leader of a UK party. Agreed. Although we vote for parties, not presidents so who leads is not relevant.

Well then what does it matter who gets to vote? Can as well flip a coin by that logic.

Of course the person matters, and whoever leads the party is in fact the face of the party. That's why election failure was attributed to Miliband and Corbyn is considered "unelectable". In theory it shouldn't be about the person, but it very much is.
Hence, the Tories should really call for a snap election, in my opinion. But that's not going to happen, because as you rightly said, we vote for parties not people here, and therefore there's no requirement to seek a renewed mandate. So they're not going to take chances.

[NotAllThere]

The biggest disgrace is to let those who live outside the UK vote in official elections or referendums.

Perhaps instead of disenfranchising after fifteen years, it should just be loss of citizenship.