OP are you sure about the 777 ?
When I booked my flight ( in 6 weeks) my choices were a while back just the 777 for the two flights (one arriving EWR 1pm and the other at 7pm) then they changed to 777 early and 787-9 later and then again to 787-9 and 787-8
Now I am on the 787-9 in first (spend some time on the phone as they first kept my flight time and downgraded me into CW).
Long story short check your seats/aircraft/.... Once in a while as BA didn't inform me of the change at all