It seems to me that freight operators in the UK have never been very enthusiastic about electric traction. Maybe this is comparable to the way Conrail phased out electric traction for freight on the NEC back in the day. If you can't run electric all the way you need to switch traction at some point and that is uneconomic and causes additional hassle. Electrification of the UK network is still rather incomplete and outside of London and the Southeast, mostly only passenger mainlines are electrified (and not even all of those) and as soon as you meander off onto secondary lines (as freight typically does) you are in unelectrified territory.
For example, the port of Southampton is AFAIK the UK's number one port for freight, and is served by quite a few freight trains, intermodals but also automobile carriers and other stuff. The lines into Southampton are electrified with third rail DC and electric locos capable of working freight on the third rail are available (classes 73 and 92) but despite there being members of both classes surplus, the operators have never expressed any interest in using them but have preferred to use diesels.
Prior to Beeching there was a very nice rail line from Didcot via Newbury directly into Southampton docks. This was the fastest route from places like Birmingham to the south coast while avoiding London. It served Highclere by the way, which back in the day had a station of its own. This is the location made famous by the Downton Abbey series. The line was upgraded at great expense during WW2 because it was judged vital for the war effort, and was thus still in good condition and double track throughout when closed. Instead freight trains today need to joggle for space along the congested dog leg that goes via Basingstoke and Reading and where freight movement is strictly limited by the intense passenger service, and which effectively prevents further growth of freight on this important corridor. The direct line is unlikely to ever return as bits of the trackbed have been used for highway projects.