If they are smart, they will pick someone from a place where Anglicanism is actually thriving, which really means one of the African bishops. The African, and other non-western diocese increasingly feel as if they are being ruled from afar by people who barely have any interest in Christianity anyway, which is bad for the Anglican Communion worldwide. And even in the UK immigrants are more and more the backbone of parishes.
The other issues is that more and more domestic bishops seem to believe the way forward is increasing middle management while eviscerating parishes. Welby was the epitome of that sort of corporate management approach.