I have just had a similar deal, with a sting in the tail.
Flying WT+ to MIA I received, by name, an offer on "Manage my Booking", an upgrade to CW one way for £339. Tried to book on line but kept being told that there was a fault on the site and to call for this offer.
When I called I was told that I did not qualify for this offer. Can't work out why other than possibly because I purchased the ticket with shareholder discount. However, going back to the Manage my Booking site, I found that if I chose to upgrade both outbound and return (the return upgrade price was £1,820) then it would let me upgrade the outbound at the low price. No question on how this was originally ticketed.
And no, i did not think this was a deal I wanted.