My best bet as to why Nigella's egg yolks are so yellow is that she's using eggs from hens that have been fed canthaxanthin. Canthaxanthin (a carotenoid) is approved for use in hen and salmon feed within the EU and is primarily used to make farmed salmon (who are otherwise deprived from their natural sources of carotenoids, i.e., krill and shrimp) look pink and those egg yolks look so orangish yellow (because egg producers have learned that customers want hyper-yellow yolks).
See:
http://ec.europa.eu/food/fs/sc/scan/out81_en.pdf
Note: I don't think that the use of these additives is allowed in the U.S.
Incidentally, eggs coming from hens that have been fed canthaxanthin are marketed where I live (in Sweden) as extra healthy due to the elevated levels of antioxidents they contain.
Now, there seem to be only positive benefits stemming from the use of carotenoids in eggs and salmon but I've come to see those shades of yellow in eggs as a sign of manipulation. Fillets of wild, Baltic salmon are on sale at my local market placed directly next to farmed, Norweigen. The blaring pinkness of the farmed salmon is obvious and, compared to the muted pink of the wild, almost obnoxious. This past summer, I watched a farmer pluck a dozen, still-warm eggs from under the bellies from a dozen nonplussed hens before selling them to me. The yolks were dull-yellow when they saw daylight minutes before being made into a magnificent, eggy breakfast omelet.
Years ago, salmon was varying shades of pink, egg yolks were varying shades of yellow and no one thought much of it. Now, in a northern Europe full of hyper-pink salmon and neon-orange eggs, I'm busy looking for the dull stuff.