My rough calculations are as follows feel free to point out any errors in the methodology.
The maximum fuel capacity of a 744ER is 241,000 litres according to Wikipedia.
The last published price via IATA of a barrel of Jet A1 is $98.9 so lets say $100 to round up.
There are approximately 159 litres per barrel of oil or in this case Jet A1.
So the maximum fill capacity of a 744 ER would be 1516 barrels at $100 each or $151, 600.
Assuming 416 passengers in a 3 class configuration gives a total cost of fuel per passenger of $364.42.
So the total surcharge would have to hit £230 before it could no longer be justified to pay for fuel.