Just a newer nicer seat on a newer, nicer aircraft really. Although for me, the difference is actually bigger in J than in F, as J on the A380 means a chance of a top deck window seat, with therefore the added space from the side bins.

Value for money / comfort and experience wise I think that for me, this exact seat, on either the A380 or B747 is the best that BA offers.

I've flow F on all three though, and I was perfectly happy on my one 777 F flight (which I think was actually to HKG iirc)