There would be savings but only marginal savings in terms of energy efficiency, and since we already have the hybrid equipment we can't trade it back to the manufacturer to get our original spending back.
However if you had a rolling program focusing on the regional routes with the InterCity services picking up the benefits of electrification when it exists you get the best of both worlds. The regional/local fleets become EMU's and the big savings that brings whilst the InterCity services just switch back and forth depending on where the wires are. With the simple to do infill of mostly non-junction lines, often with few difficult to change urban bridges, used by the InterCity services between the urban areas potentially not needing many (if any) feeder points, meaning that cost of doing so would be lower per mile than urban schemes.
It means that to convert another 30 units to electric power is easier to do than wiring up hundreds of miles and increasing the under the wire running of the local services crossing it or running along the route for a bit. Rather 50 miles could be done, 20 of which the InterCity services use, 30 trains get converted to EMU. It may cost more per mile but the number of units, maybe even the number of coaches, per 100 miles would be higher.
As an example if you wire up around two nearby cities and the main route between them totaling 50 miles. Then loads of regional and local services an hour would benefit (say 10tph with 3 coaches each=30 coaches) whilst to convert the InterCity services through those same cities would require 100 miles of wires but it would benefit less coaches (say 2tph with 9 coaches=18). If the costs for the urban electrification was the same (say £0.5bn), the cost per mile is double (£10 million Vs £5 million) but the cost per coach changed to electric is much smaller (£17 million Vs £28 million).
It would also improve the air quality in those cities, whilst the pollution would be in the countryside where it can better cope with it (not least as there would be much less of it).
It then allows for the next city across to be wired up repeating the process and allowing it's services (and possibly more of those from the two first cities) to go over to being electric.
Yes some cities would need to be looked at in stages, such as Birmingham. However if the likes of XC have bimodal trains for a long time to come to cater for running through the likes of Cornwall, what's it matter to them if there are gaps elsewhere?
