It sounds like matt_likes is happy to pay whatever to get a J class seat. In that case, as you say it's a Y flexible fare, once the ticket is issued, he should be able to use the "Upgrade" function on manage my booking, even if there isn't a special offer?
Or does this not work if the ticket is booked through a travel agent?
UuA may do, but depending on the precise sales channel I would not expect the Upgrade button to work since the ticket is still owned by the travel agent until T-24.