Config changes - I picked a mid config B777 which was two class. As per burn, I am not a 777 guru, but have asked guys what their burn has been in cruise and 'generally' the answer is 4,000-5,000kgs/hr. I can't recall any higher. Of course that's in cruise, a 744 burns 40,000kg/hr on the takeoff roll
I can quote this which was posted here a few years ago as a specific flight in 2010, an EK 777-200LR carried 98t of trip fuel (Flight Time: 13h55m). I didn't make a note of the load (if it was even available), but that indeed shows a higher figure of c7,000kgs/hr.
So to make the adjustment that's 'around' an extra 85,000kgs one way to LAX (170,000kgs one way to LHR) for 'around' 100pax extra. Better hope they fill those 100-odd seats or it gets VERY expensive