If Exeter is the furthest west point that would be able to cope with the demand on the grid though, you wouldn't be able to feed Penzance from Plymouth and I'm not sure how well feeding Penzance from Exeter would work.
Right, I have found what I think is the longest single end fed section on the network, I make it 28 route kilometres between Saltcoats feeding station and Ayr on the Ayrshire Coast line (although it is isn't
technically single end feeding in the normal sense, it does not seem to be able to draw on another feeding point as far as I can tell with second place going to Skipton being fed from Bingley feeding point at roughly 20 route kilometres.
I shall resume plotting and report back soon.
EDIT:
If we use the gap between Marshall Meadows and Ulgham Crossing on the ECML as an example of pushing the gap between substations to the limit (as we all know the ECML was bargain-basement, although I still defend the quality of the workmanship). That gives us a figure in the vicinity of 78-80km of route between two substations.
This means that if we assume there is a substation in the vicinity of Bristol TM (which would make sense as it can support both of the routes to Bristol and the SWML from that position) we would need a feeding point somewhere just west of Taunton.
Taunton to Exeter is too far for single-end feeding in near main line conditions (or atleast Ayr Line conditions) so it would appear that a new feeding point is required anyway, which could be positioned adjacent to Exeter St David's station.
Unfortunately I don't think single end feeding Plymouth from Exeter is practical (due to it being 70+km) requiring another point at Plymouth, additionally electrifying the WEML using this settup would require only one new feeding point, one somewhere just east of Salisbury (and would be needed for electrification to salisbury anyway assuming 25kV with switchover at Overton). The light traffic on the line and its low power nature (even a six coach EMU would probably draw only 2000hp peak) means it could be fed from Salisbury-East and Exeter.
Feeding Paignton is a challenge but the branch is sufficiently short that it could be single end fed via the branch by Plymouth and Exeter feeding points.
It would appear that there are major effects that mean once you commit to electrify beyond Bristol you have to go quite a ways......
Placing a point at Plymouth (if we assume its the furthest west we can go without seriously unbalancing the local grid) would give us the various secondary lines around there, but the branch lines radiating from around Exeter would require relocating the Newton Abbot point into Exeter.
EDIT #2: I note that the 2009 NR Electrification RUS includes the line to Plymouth and Paignton in its "Cross Country" scheme.... the one with the BCR of 5.1/3.4 (the latter includes the Colton Junction-Leeds section which I believe has already been commited as part of the North Transpennine project?)
Isn't BCR of 3.4 above the one normally given as the cutoff value? (2.0?)
WEML comes out with a BCR of 3.1 too.