But those services aren't electrified anyway.
Agreed at present. Saying "no overheads in Bath" so leaving it with diesel or other technology lower distance trains into the medium future marginalised things like the Bristol Metro plans and so many other places accessed through Bath - London services are / were / would be / could be just the first to use the overheads.
I think there's a long-term plan or ambition to electrify Southampton to Bristol? Not electrifying through Bath is clearly incompatible with this, as you say.
There's a long-term plan or ambition to electrify the vast majority of passenger and freight mileage, Network Rail's Traction Decarbonisation Network Strategy (
https://www.networkrail.co.uk/wp-content/uploads/2020/09/Traction-Decarbonisation-Network-Strategy-Executive-Summary.pdf). Unfortunately the government has not (as yet) signed up to implementing it. The only service through Bath not earmarked for full electrification is the Bristol-Weymouth route, which I think is proposed for battery operation between Castle Cary and Weymouth (or possibly a little less if the units can use the 3rd rail). I fear the government will continue to refuse to fund electrification and that, as a result, the rail industry will lose it's 'green' credentials; still having diesel trains in 2050, long after new petrol and diesel cars are banned, will not be a good image.
The problem would be the business case. If the only use made of the wires were by a limited IEP service then too little justifying benefit would be indicated. Perhaps if the Cardiff - Bristol trains were turned over to an EMU/EDMU shuttle and the Taunton's and Portsmouth's were run together, then there might be a case.
Will there always be a viable business case for decarbonisation? Sometimes, we may have to do things that don't stack up in BCR terms. That said, if we can make the BCR for electrification look better we should, as it makes it more likely the treasury will authorise it. For example, if you have an elderly diesel fleet that is due for replacement the cost of new EMUs can be partly written off against that; 'we need to buy new trains anyway'. Conversely, if you have DMUs with at least 15 years life left in them there's no business case for replacing them. I'm trying to get the Welsh Government to understand this regarding the large new diesel fleet TfW has ordered, but they don't seem to want to know and just keep blaming the UK Government for the lack of electrification in Wales.
As for an EMU shuttle between Cardiff and Bristol... Personally, I think the Cardiff-Portsmouth should stay, worked by new tri-mode units taking the best design elements from 158s, 175s, 442s and 444s (or possibly 800s if London demand has fallen so much that fewer are needed). However, I would replace the Cardiff-Taunton with a 2tph Bristol-Cardiff shuttle (thus increasing the frequency between Cardiff and Bristol to 3tph, with the shuttle calling at many stations and the Portsmouth being a fast(er) service) and send the Taunton services up to Gloucester/Malvern instead of Cardiff.
