Posted on WNXX late last night by a reliable GWR employee "Ingleborough"
"This is what came out during a meeting at work this afternoon.
Temple Meads via Bath and Chippenham will not see wires for a long time. No provisional date could be given. The only route fully wired by December 2018 under the latest revision is London to Swindon to Bristol Parkway. There is not even an estimate for dates to reach Cardiff, Swansea, Oxford, Newbury and Basingstoke. Thames Valley Branches not even mentioned.
Temple Meads redevelopment will have to be finished before electrification reaches it, and that means the Filton bank work has to be finished at the same time.
So it will be pull into Swindon or Bristol Parkway, pan down, fire up the MTUs and carry on at a reduced speed."