This is a particularly interesting thread with regard to the mass emigration information, which I will follow up - thank you very much for the details of where to look.
Strangely enough I have a Miller family emigrating to New Zealand in about 1874, although I don't think connected with yours. James Miller (1821 Middlesex), Mary Ann (Clifton) Miller (1821, Seal, Kent), and two teenage children James G Miller (1858, Seal, Kent) and Mary Ann Miller (1860, Seal, Kent). They went to Invercargill, as I have been able to establish.
But it's fascinating to find out why they might have gone!
very many thanks