I don't really think it can fully be blamed on privatisation though. "It is presumably cheaper for privatised train operators to employ diesel traction on services which start or terminate beyond the confines of the electrified system" well if its cheaper for them surely it would also be cheaper for BR?
Allied to my first point, I think it would me more accurate for the quoted statement to read "
smaller, privatised train operators". British Rail was of course a large single entity whose railfreight sector had a monopoly on the market, and consequently could afford to maintain sufficiently large fleets of diesel and electric locomotives to achieve economies of scale in their maintenance and operation. The same goes for having a large pool of train crew to select from, with a range of route and traction knowledge.
With the fragmentation of the railfreight industry, smaller operators such as GB Railfreight don't appear to have the critical mass (though they have gone from strength to strength!) to support both a large fleet of diesel and electric locomotives. It is no longer the case that traincrew from a depot in any corner of the country can be called upon to work any potential railfreight flow, as company drivers will only have the route and traction knowledge pertaining to their FOC.
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it(anecdotally) isn't that high on the southern end of the WCML
Compared to the ECML, I would actually say that the WCML is a poorer example as a bastion of diesel haulage under the wires. Since the start of regular through running of London-Birmingham-Scotland services, there are fewer Voyagers operating Birmingham to Scotland services, with them being confined to a greater extent to North Wales Coast and off peak Birmingham to London diagrams. TPE Manchester to Scotland services are now mainly electric 350s, and the likes of the TPE Barrow services are only under the WCML wires for a short distance, between Preston and Carnforth.
Compare this to the ECML, which sees East Coast HSTs operating the full length of the electrified route quite regularly on both longer distance services and some "captive" diagrams that remain fully under the wires. Two open access operators, Hull Trains and Grand Central, operate solely diesel trains over a considerable length of the southern section of the route, and there are further East Coast HSTs utilised on shorter distance "off wires" services to the likes of Hull and Harrogate. North of Doncaster/York, there are also two diesel Crosscountry and one (two as far as Northallerton) diesel TPE service each hour.
Electric freight on the East Coast main line is largely non-existant as well, while on the West Coast main line classes 86, 90 and 92, as well as Royal Mails' electric postal units, are regular sites throughout its length and particularly between Rugby and Wembley.