Bristol-Derby went to the back of the queue when the route got new diesel trains (Voyagers). Just like The Snow Hill lines when they got their 172s.
GW is being electrified mainly because the HSTs (and signalling) are life expired and have to be replaced anyway (plus the Thames Valley diesel fleet is overwhelmed).
MML is an odd one, because its fleet is not life expired (not the 222s anyway).
Every electrification project has to look at the cascades as it is key to the business case.
What would you do with the Voyagers?
Bristol-Derby will keep failing that test until more of the XC route is wired by other projects.
The coffers are only so deep anyway, and NR and contractor resources are fully stretched with the current work.
It may be a long time before the next major route electrification is approved (maybe the 2017 HLOS).
Unless Mr Osborne can produce a rabbit out of the hat in the next 6 months, to pacify the Tory marginals.