These are very good questions. Feeds in to the discussion of the various Newcastle locals above as well.
It's a problem - the more you know about a situation, the more complicated it becomes!
The Newcastle area local stuff is frustrating - if BR had had a couple of quid more thirty years ago then they might have been able to do the Hexham - Sunderland (and the little section at Morpeth) but now that the Metro runs to Wearside, that removes the benefits you'd get for wiring as far as Sunderland, which takes the wind out of the sails of any other proposals (and makes them too small scale to get a team together for?)
That's why I'd prefer operationally self-contained services rather than doing things for the politics of it.
The reason I picked the Calder Valley Services out of Leeds is something you've pretty much encapsulated, except that Leeds to Manchester via Brighouse will soon (in railway terms) be partly electric. A year ago you had services running York - Blackpool, Hull-Halifax, Bradford-Huddersfield, Burnley-Manchester, Leeds-Brighouse-Manchester Vic and Leeds-Halifax-Manchester Vic. So that would need a fleet of bi-modes - more likely one of the Scrapheap Challenge fleets that keep not quite arriving in service - or you do all of the bits of route off the main route as well.
Politically you'd do it so you could talk about "modern" trains for Burnley, Rochdale and Bradford. Operationally, forget it and do something simpler. And cheaper.
You make good points there - there are potentially a few lines where we could electrify but we'd need to simplify things to make the economics work - i.e. it's not efficient to wire it if we kept the same service pattern but the frequency of trains could make it worthwhile - e.g. there's five trains per hour from Sheffield to Doncaster (a similar distance to Uckfield - Hurst Green) - but would people round here accept the benefits of electric trains if it meant losing the direct services through to places where the frequencies don't warrant wiring (e.g. with just one train per hour to Cleethorpes, it probably wouldn't get done, but then that'd either mean no through trains from Sheffield to Cleethorpes - but if we kept running DMUs from Sheffield to Cleethorpes then that'd take away part of the benefits of wiring the line to Doncaster since not all of the trains would be switching over to the new power source).
What makes the cost so high?
I can't speak with any authority, but I guess that you'd have to consider the tunnel(s?) on the route plus you'd be spreading the costs of the project over a could of dozen miles (which, given the set up costs, the "getting a team together" stuff, might make the cost per mile prohibitively expensive)
Some electrification projects can be justified due to the air pollution in urban areas (which isn't much of an issue at Uckfield), the regeneration of poorer places (which isn't much of an issue at Uckfield), sometimes you can justify investment in "swing seats" as far as politics goes... but I'm struggling to find something that would push the Uckfield line higher up the list than it currently is,
assuming that we can get battery/hybrid technology that can cover a fifty mile round trip at relatively low speeds, with the train charging up again north of Hurst Green
I don't like the idea of "diesel islands", I wouldn't be unhappy if the two Southern routes were electrified, but there are economies of scale with bigger projects, which means that a relatively small bit of line is disproportionately expensive
