Depending on your dates (i.e. not 10-24 Dec) i've now found that the downstairs back cabin is best.

There is a bunfight for the upstairs seats so it ends up packed. Check-in allocates from the front back, so 3 / 4 of my last 380 flights i've had a row of 3 (day flights) or 4 (night flight) to myself. And it's obviously dependent on it not being a full flights.

But at 84 vs 94 seats the size of the cabins are not that different, and i've seen about 10 seats taken in my cabin vs the upstairs being almost 100% full.

Mind you I thought they were removing the Y deck upstairs A380's from LHR? Guess they have changed their mind a few times.