As I discussed in a previous blog, the most practical way of running trains over routes which are only partly electrified is to do what was done on the Waterloo to Weymouth line when the electrification ended at Bournemouth.
A suitable fleet would be composed of electric locomotives or powered sets, and trailer sets. The powered sets would have a streamlined driving vehicle at the London end for 125 mph running (which might be an electric locomotive) and a gangwayed (possibly, but not necessarily) driving vehicle at the country end.
The trailer sets would have a streamlined driving vehicle at the country end, for 125 mph running, and the gangwayed driving vehicle at the London end, something like a class 375 Electrostar.
Down trains would be propelled from London to the end of the electrified route and would then split, with the trailer vehicles being hauled to their final destination by a diesel locomotive and the electrically powered portion remaining to form a return working to London.
Up trains would be pushed by the diesel locomotive (ie blunt end first) to the point where electrification began, where they would couple to the electrically powered portion of the train. The diesel locomotive would be detached and the train would continue to London under electric power.
Is this so difficult?