I'm flying the A380 next month, and so far it appears that all the good seats are taken. Do BA block certain seats (e.g 53A/K) for advance reservation, or are they truly occupied?

On westbound flights I always chose the K side, since I would otherwise have the sun burning me. But in this case since I'm flying LHR-SFO BA287, the flight departs at 3pm. Will the sun set right after departure, or will the daylight remain through the flight? I'm flying on xmas eve.