Of course, but that doesn't alter the fact that dead weight is still being lugged around from London-Bristol Parkway (or wherever). That said, if one attaches a diesel locomotive at the point where electrification ends, then a load of useless traction motors would still be being lugged around if the train was a 'pure' EMU.
This is why I still believe - as does Ian Walmsley in Modern Railways (and even to some extent Roger Ford) - that when a train needs to change power source along its journey, a fixed formation of carriages with a driving cab at either end is the best solution. For the GWML, something like this would be quite neat: electric locomotive at east end of the train, pushing it to wherever electrification ends. Upon arrival at the boundary station, rear loco drops off, diesel loco couples onto front, hauls train to final destination. For the return journey, the reverse applies. This minimises 'dead' weight and offers optimum flexibility.
Okay, so for arguments sake lets say the electric set is a 225 and the diesel engine is a 67 (it won't be a 225, of course, but they are a working example of what I think you are suggesting)
So the 91 pushes the train from London to Cardiff (where the electrification ends). At Cardiff the 91 detaches (a separate driver going to the rear of the train to drive it away?) and runs off into a siding/bay. The 67 then comes along from another siding/bay, the driver changes ends and the 67 pulls the train to Swansea
(apologies if I've got anything wrong, let me know...)
Has anyone got an idea of how long this manoeuvre would take to do? I'm asking as an open question, I'm not trying to belittle, just to get an idea.
I grew up watching the diesels taking portions from Edinburgh to Carstairs, and attaching the train to an electric hauled service from Glasgow down the WCML, but I have no concept of how long it takes to uncouple, drive the 91 away, drive the 67 in, attach it and drive it off.
Presumably passengers will be okay to board/unboard whilst all the shunting is going on.
However, there will be complications with the movement of the 67 and 91 - additional platform/sidings required, additional paths needed at busy stations etc
I don't think there's an ideal way of doing this, we are either going to have to put up with "diesels hauling electrics around/ electrics hauling diesels around", a train with unecessary weight or cutting a lot of direct trains to London from GWML branches
(of course, this wouldn't be a problem with MML electrification, because 99% of London trains never stray from the core route - only a token daily service to Lincoln and Melton Mowbray and a Summer Saturday extension beyond York to Scarborough would need revising - everything else is self-contained)
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