BA newbie here. Booked first BA flights SFO-LHR for next spring on A380 in J. I am a bit concerned about the backward facing seats and wonder if I should spend the extra money to select seats in advance - I have never flown an airline demanding a fee to select a seat in J before, and I don't have oneworld elite status to select in advance. My questions are:

1. Does it feel any different sitting backward, in terms of motion, etc.? Would the "average" person notice any difference? If so, does it differ on main deck vs. upper deck?

2. Any idea what % of seats are usually left for free assignment at T-24? And, of those remaining seats, do they tend to distribute evenly between backward and forward facing ones, or do they tend to be mostly backward (or forward) ones?

Thanks.