I really cant see them using a 767 to NCL. The UK market is perfect for the 757, keeping the 767s free to launch routes further into Europe.
If they are struggling for 757s, then it would make more sense to upgrade an existing 757 route that is near full capacity, rather than using one to Newcastle. I think they send a couple of 757s to Heathrow still, this must be the prime candidate for an upgrade. Either that or Manchester/Glasgow.
Newcastle would surely be near the bottom of the list for a 767.