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I cycled to the Humber from London once with a section of the route burner from London-Edinburgh-London.
A more detailed search might find more for you, but there's some parts of the route here: https://ridewithgps.com/events/18751-london-edinburgh-london-2017#routes/23461128/preview
Maybe that would be a good start? Although in my head, you've already ridden that.@giofox seems to have the right idea with Angel2Angel.
Anyone got a tried and tested route from London > Newcastle?