As someone who has suggested that Wesbury to Taunton doesn't need electrifying until after the Cornish main line as services between London and Cornwall can run via Bristol I would like to explain why.
Firstly IC125's are not fine for another 20 years, as although the engines may last than long and the coaches COULD theoretically last that long, they would need a fairly expensive rebuild and replacement of some fairly critical items. Even then some of the coaches may need to be scrapped before then because of corrosion. Which is likely to make it not cost effective to carry on using them much beyond 2020. See this thread:
http://www.railforums.co.uk/showthread.php?t=74368&page=1
I was not suggesting that once the Cornish main line was electrified that there would be no services to Cornwall along the B&H. Rather there would be SOME EMU services to Cornwall which ran via Bristol. However the majority of the services would still run along the B&H as Bi-model.
This is because it is better to not electrify the B&H and run Bi-model trains along it and electrify in Cornwall first. As by doing so many more services can be converted to EMU running (including the XC services) and there would be enough services for NR to justify looking at line speed enhancements between Bristol & Exeter.
Also if the B&H is electrified and electrification reaches Plymouth, then if in the future the government isn't investing as much and doesn't wish to electrify more; then Cornwall could no longer have a direct service with everyone changing at Plymouth. This would be to avoid running long distances under the wires or have having a new bi-model train built. However if B&H is done last and the government doesn't wish to electrify more then the B&H will probably still be done as it would be cheaper than having to build a new bi-model train.
A likely service pattern would be 1 EMU service per 2 hours (9 coaches) running via Bristol with similar journey time as via the B&H. Then 1 Bi-model service per hour running via the B&H made up of 2x5 coaches which splits at Plymouth. Either with one set retuning to London whilst one runs onto Penzance or with one running to Penzance (semi fast to Par) and one to Newquay (stopping).
On top of that there could also be a new hourly bi-model stopping service from Paignton to Paddington along the B&H (semi fast from Westbury to Paddington). Which could remove some of the passenger loadings from the Cornish services towards the eastern end of the route.