OK, but surely the voyagers are becoming less efficient and in 2020 it will make economic to replace them with "hybrids"?
I think Bimodes is the term you're looking for, although they technically are hybrids, they do not operate in the manner that is normally referred to by hybrids, 80x units (as yet) have no ability to reuse any energy produced via diesel power.
Yes indeed, as regular DMUs with very large engines Voyagers are quite inefficient. That alone, however, does not justify their replacement - the cost of acquiring new units is considerable - the bi-mode 800s came in around £2.8 million per vehicle.
It makes sense to me that if the ECML intercity operator is nationalised, so too should be the WCML operator, especially if HS2 ever goes ahead - the franchising system seems incapable of avoiding a monopoly, but that's far less an issue if the monopoly is the government.