Although, WRT the LHR/JFK route my (admittedly not vast) experience has always seen packed flights on this route. If flights are consistently packed to the gunwhales then I can't help feeling it suggests there is more demand and BA could, in theory, sell more seats...hence an A380 might do the trick.

Ultimately, BA know better than any of us which routes might merit an A380 - and it might be that BA want them, but aren't willing to pay Airbus' price ?