Thanks for the replies folks.

Yes, I indeed do have two one way bookings, rather than a single return booking.

It's actually not so stupid forgoing a return leg, as if you look at the pricing structure of taxes & fees for any flight, the vast majority of the costs are incurred on the UK-> US leg. The US -> UK leg, only costs £72 in taxes/fees, so this to me is worth it to upgrade from J to F

Kevin.