The length of the runway tarmac is only partially a factor, you also have to consider the position of the thresholds, are they displaced as this will reduce the take off run available, but the big killer is obstacles and the ability of the aircraft to climb in the event of an engine failure.
I image the latter is the reason for restrictions at BHX given the many obstacles in the take off flight path.
That said, an A380 to Dubai is going to be no where near it's max take off weight as I should imagine the limiting factor is the zero fuel weight and so I can't imagine that for 6-7 hour hops it would make that much difference, but then I don't know what the OEI performance is like on the ugly bus