Some say the B787 is 25% more efficient than an equivalent ?
How much better would a Boeing B787 be on a route, for instance: Manchester to Dubai, Abhu Dhabi or Bahrain?
Compared to an Airbus A330-200HGW ?
That depends entirely how you define "efficient".
One measure might be fuel burn per seat-mile (or fuel burn per aircraft mile, for a similarly sized aircraft).
Nobody, least of all Boeing, would claim that the 787 is 25% more efficient in that respect, compared to the competition.
Would the aircraft cost (assuming the Airbus was either second hand or bought/leased at a preferential rate) Be part of the equation ?
What about operating costs ?
Obviously acquisition cost would form part of an potential operator's overall comparison exercise. But comparing a new-build 787 with a used A330 would be a fairly pointless exercise.