If there was a sacrifice to be made and a UK 757 had to be cancelled - I'd have chose the NCL flight.
The other UK flights are established and will contain regular and loyal customers. The NCL service won't, as it's not guaranteed past this season, and hasn't really had long enough to create that base.
The conundrum is... do you try to impress a potential new customer or upset an existing one.
I know which I'd choose.