theredbarron,
I don't know where you learnt your maths but on 'long,thin' routes such as UK-YHM, the 757 is a much more efficient machine compared to the NG. It can easily make YHM from any of the mentioned UK departure points non-stop whereas the 737NG struggles hence all the tech. stopping in Kef' or St. Johns. And that's before you factor in the ferocious North Atlantic headwinds.
It may burn slightly more fuel but carries about 20 more pax (bonafide Buisness class hi yield), negates a costly tech stop for fuel therefore reduces possibilty of tech problems and doesn't expire the life of the aircraft as quickly as the airframe doesn't have to pressurise & de-pressurise.
Boeing designed this aircraft for the exact type of route GSM want to operate in UK-YHM.