I think the obvious place is to Northern on pacer replacement, although much of northern's network is slated to be electrified (i.e. Harrogate line, Leeds-Sheffield & Caldervale, plus stuff in Lancashire). The obvious answer is pacer replacement, so move to Northern, allowing 150s and maybe 158s away to FGW & ATW.
I think it would make more sense to cascade additional Sprinters to Northern, rather than setting up one small new fleet of 170s. Plus, at the current time electrification of the Harrogate circle, Leeds - Sheffield and the Caldervale are all only aspirations, and with electrification of TPE North in some form or another committed to between 2016 and 2018 (provisionally), then it is unlikely that we will see wires going up over those routes you have stated before the CP6 period next decade.
Also, as
jcollins says, how do you propose to both replace some Pacers and also allow some Sprinter units to be cascaded with only twelve 170s?
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What's interesting is that NXEA, in their RUS responses didn't see the potential of electrification of those lines, but saw potential in the Southern 171 routes being electrified and suggested if that was to go ahead they could get the 171s and release their Sprinters to Northern as Pacer replacement.
Sounds like a sound idea, and is pretty similar to what I would advocate. Although I would transfer the 6 x 4-car 171/8s to TPE which would replace the 9 x 2-car 170/3s: Once the North West electrification is complete, then TPE will have sufficient stock to allow the 170s to return to operating primarily in pairs, so six 4-car units would be sufficient to replace nine 2-car ones. Plus I have seen it reported that it would be difficult to convert the 171/8s into 170s, as they were built as 171s to have compatability with 377s and their software and electronics is incompatible with 170s, while the 2-car 171/7s started life as 170s in many cases and can easily be converted back. The 171/8s would also retain their Dellner couplers so they could be coupled to a 185 in an emergency, even if they couldn't work together in normal service due to the different software used.
The 9 x 170/3s and 10 x 171/7s (Converted back to 170/7s with BSI couplers) would then go to GA to displace their 5 x 153s to EMT and 9 x 156s to Northern. EMT then lose their 15 x 156s to Northern, in exchange for Northern's 19 x 153s. Admittedly this maths worked better when EMT had only eleven 156s (With Northern losing 19 x 153 carriages but gaining 40 x 156 carriages, and EMT losing 22 x 156 carriages but gaining 24 x 153 carriages), so I suppose it would have to be hoped that EMT would also gain London Midlands' 153s through some other rolling stock development in order to match EMTs' current capacity. Northern would then have a larger fleet of 156s and no 153s (3-car services previously formed of 150+153 or 156+153 will become 4-car formed of 150+156 or 156+156), while EMT would have a standard fleet of 153s sufficiently large enough for them all to be formed back into 2-car 155s.