You won't see anything online given that routing, see the POUG thread for more information about that. But if it's a general question then yes, BA will cheerfully leave the cabin empty rather than cannibalise their more expensive fares. On return trips, as an extremely general rule, it's usually best value to buy at the initial date of purchase, since POUGs are entirely bargains. If you had a one way ticket that would be different but on a return ticket then one-way First usually can be accommodated within the fare basis of the remaining CW sectors.