I think it's a combination of economics and politics. Post WWII the government of the day wanted to electrify, and the Sheffield-Woodhead-Manchester route (now closed) was actually the pilot project for a network-wide electrification. As a country we simply couldn't afford it at the time, and we ended up with the 1955 Modernisation Plan which was intended to bring the railway system up to date, introducing diesel engines to replace steam as a stop gap
before going to full electrification. The aim was to help eliminate BR's financial deficit by 1962...
As part of this plan it was envisaged that the principal main lines, in the Eastern Region, Kent, and the London-Birmingham-Central Scotland corridor with Glasgow to Edinburgh trunk services electrified as well. In the end we got London to Crewe, the Glasgow area electrics and London to Clacton as the first stage whilst everything else was dieselised, because siuccessive governments either couldn't afford it (or in the case of some Tory administrations wouldn't afford it). We still have this mess today, with the debacle of Northern Powerhouse Rail being a primary example of Treasury parsimony
I thought the Cumbrian coal was primarily wanted for specialised steel production in Sheffield. Like it or not we are dependent on steel for some items (e.g building frameworks), and steel production requires carbon in the form of coking coal. At least one source states that 85% of the coal mined in Cumbria would be exported, generating much needed export earnings as well as improved employment prospects in an arae which has been in industrial decline since the closure of the steel industry (Workington rolling mills in particular) and the downturn in shipbuilding (Barrow-in-Furness) from the 1970s onwards. Low-paid seasonal tourism jobs can never replace properly paid employment. This becomes an even hotter topic when you realise that 40% of the UK's coking coal currently comes from Russia
, the balance being imported from Australia and North America. Shipping all this coal around generates a massive amount of carbon dioxide, one estimate putting the UK's cvarbon footprint for importing coal at 1.28 billion tonnes a year, which could be drastically reduced if we produced coal in the UK. The size iof thge issue is highlighted in this article