In terms of the runways used at LAX, assuming it's physically possible to get the A380 to the southerly runways, why would you not use the extra 600m of 25R for such a heavyweight departure? Surely a few mins extra taxiing is going to make little difference to block time on such a long flight?
Wycombe, taxying at LAX is tight for a B744 far less the A380. The A380 has four "Safety" vehicles escorting it when it's underway at LAX and SMC has to hold up other aircraft to give sufficient clearance on some taxiways. ATC and all the other operators appreciate it when the A380 accepts 24L for departure.