Personally, go with the 787.
Smaller cabin, rather than 2 'dorms' and better/faster service.
You're in the top cabin so no nagging thoughts about an upgrade.
A380 doesn't have much going for it apart from the extra storage in the UD window seats (and the massive lavs on the UD). LAX is an MF crew which, some have suggested, means a slower service.
You'll arrive at LHR in time to use the Arrivals lounge, which will likely be closed/running down the food for the LAX arrival.
Just some thoughts.