Booked my first F flight for May JFK-LHR (early evening) and looking forward to it!

When I went to choose my seats 1A and K were blanked out for Golds (I'm Silver) but the rest of the cabin was empty.

What's ppl experience of the number of final F passangers on this kind of flight?

If the cabin is not fully booked at the time of the flight do they upgrade J passengers to fill the empty seats if they've over sold economy?

Thx