Economy on BA (applies only to WT not any other class of travel):

Never fly on a 787.
Preferably fly on an A380 (as you are).

If on the A380 sit in the upper cabin towards the rear in the exit row window seats. Board early because overhead bin space is limited back there, but it's quiet and the exit row has great legroom. The doors and slides and so on are not a problem.
Bring some snacks and a large bottle of water, while the main meal is adequate other things are not, and the breakfast can be dire.
Bring a smaller bag you can put by your feet with in-flight needs, to keep your main bag in the overhead bin (especially on the A380 at the upper back section).
Bring something to rest your head on and provide neck support; a folded fleece or small pillow.
Bring your own entertainment as needed, the WT headphones are (as expected) very poor so bring your own, the A380 IFE is the same in all cabins and quite OK.