As regarding the lower fare with A380... that is assuming the plane is almost full, if it is not, there is no point talking about lower fare. In the event of economic downturn, the plane can be half empty, and the cost would not be absorbed by the 50% load. Didn't CX decided to park the planes during the SARs instead of having them in service because it would save them more if the planes were not flying.
The A380's operating costs per seat are much lower than the 747 and lower than any other aircraft flying with the same percent of seats filled. And who says the A380 is going to be half full and 747s full? Cut the fare marginally, and offer the wider seats and longer legroom that the A380 offers, and the A380 will be flying full and two 747s half empty. 747s flying half empty make a big loss. 747 sales have slowed to a trickle, 777s and A340/A330s are squeezing it at the low end, and the A380 at the high end, so the 747s, not the A380s are the ones that end up being parked on the tarmac.
LHR is a busy airport with limited landing slots. I can't see any way of scheduling more 787 flights in to try to compete with the A380, unless BA cuts it's passenger numbers and switches to flying to smaller airports. Since BA wouldn't need the high density routes through LHR, maybe they might even think of switching to London Stanstead or London Gatwick.
Naah! The more I think about it, the more of an extreme fantasy this scenario turns out to be. Maybe Ryanair won't need the A380, but in the long term BA would be committing suicide if to chooses not to operate them. BA will resist buying the A380 if it has overcapacity, but competition with other airlines flying A380s on the same routes will force it to either concede it's high density routes which are it's bread and butter or fly 747s at a loss on these routes if it doesn't match them with A380s of it's own.