This thread raises a number of issues.
Whether we like it or not, electric haulage of freight is usually quite 'awkward'.
There are several small fleets of generally old and/or technically obsolete locomotives that will typically provide poor availability because of shortage of spares, the 'only' depot being awkwardly placed in relation to some traffic flows, difficulty in maintaining driver competence and so on.
Operationally many terminals will require an extra shunt locomotive and staff to deal with final positioning off the wires that can be avoided with diesel main line traction.
Because much freight runs overnight it can be subject to diversion onto non-electrified alternative routes or face issues with isolations.
Ironically the recent growth in some freight sectors actually makes it less attractive to use electrics. For example when Daventry only saw trains to Central Scotland it could be an 'all electric' operation. But now that there are regular trains to non-electrified places like Wentloog and Tees diesels may well offer more efficient diagramming.
Some network upgrades and gauge clearance such as Felixstowe-Nuneaton and the Joint Line through Lincolnshire have led to freights being diverted off former electrified routes via London and the core ECML.
It is well known that the freight operators make very little profit (if any) according to ORR figures and so any 'penalties' for using diesel would simply lead to price rises to end customers and risk of loss of traffic to even more damaging road haulage.
Medium distance intermodal freight often needs Mode Shift Revenue Support to make it 'viable' as it is. Any increase in costs risks needing a significant increase in taxpayer 'subsidy'.
There may be sufficient environmental benefits from more electric haulage to make such government support worthwhile but it is a difficult ask when the economy is being so badly damaged by Covid.
