I'd say the lion's share of time was spent seeking planning permission, writing environmental impact documents, planning the route, building the justification for any compulsory purchases, consulting with interest parties, etc. Some of that would be NIMBYs, but I don't think there was a sustained campaign to hold back EGL2, it just took a lot of bureaucracy, and effectively "death by a thousand cuts". A project gets to be 5 years late one day at a time.
See the timeline and some of the documents here: https://www.easterngreenlink2.co.uk/project-to-date
And Nimby is politics, so I don't see a distinction between Nimby and 'purely politics'.
You used a fine phrase there, "red tape", which describes all the bureaucracy over and above localised objections to national infrastructure. NIMBYs can certainly use the red tape to hold you up, but there's also NIMBY-free red tape that holds you up anyway.
Under "politics" I'd include NIMBYs and red tape... and also political will. The project was proposed in 2015 and didn't really start until 2020. That's politics -- convincing the appropriate people in power that they can and should do something, and commit to funding the project, assigning the appropriate governmental bodies to begin work on it, and in this case engage the companies responsible for electricity transmission and get them to work on it too. Only then do they start planning the route, applying for planning permission, compulsory purchase, holding consultations, hearing objections, etc.
Getting the project going took 5 years, it could have been 0 years if the political will was there.