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If you look closely it is just "lighting"(well I think it is)... but it's lighting that's been altered to make it look like a Russian hat, against that background, etc. etc. etc.
The reason the response is bollocks, is because folks referencing the hat are using it for shorthand for the whole display.
Honestly, I think the motivation was most likely a joke that got carried away. But that doesn't change how fucking awful it was for a show like NN from a public broadcaster to pull that shit. Even more so after the spy smear. An apology would have been more appropriate.
The gif of the two images of Corbyn (original and NN) still doesn't convince me that his hat wasnt altered beyond a change in lighting making it look more Russian. In many ways it's become a distraction. The BBC does, I believe, have a bias towards the Tory party, directly and/or its own leanings towards middle/upper class being part of the establishment.