Busy main lines will have to be electrified ultimatly due to the rising cost of diesel fuel.
In the near term though I have serious misgivings about electrification due to the large scale delays caused by failures, with main lines closed all day or even for several days.
Diesel trains are of course also subject to breakdown, but the effects of a broken diesel are usually less serious.
A failed diesel train can usually be hauled or propelled by another train, or by a rescue locomotive. An HST or DMU can usually proceed at reduced performance if one engine fails.
Passengers affected can usually complete their journey either in the delayed failed train or on the next service.
A failed diesel train is unlikely to block other lines on a muti track route.
When the OHLE falls down, as seems to be regretably common, it can lead to the closure off all lines for an extended period, there being no question of continueing on a following service if the line is closed for the rest of the day.
I for one do not look forward to the coming electrification of parts of the Great Western. I forsee years of disruption with no effective service at holiday weekends, and Monday morning problems when the work over runs.
Parts of the South West are very exposed to bad weather, and the wires will no doubt come down in windy weather, and the masts be displaced by landslips in wet weather.
And of course electrification means new trains with high density airline style seating and no restaurant and limited luggage space.
And yes I know that the interior layout of the new trains has yet to be decided, but previous generations of new trains do not fill me with confidence.
Indeed did not a respected member of thse forums state "you cant expect new trains to have 1950s seat spacing" or words to that effect.
The cramped layout of modern trains is not of course a direct result of electric traction, but will be a side effect.