Interesting, I think you might actually have to talk to someone in BA/BA Holidays in order to get your answer, FWIW:
http://www.britishairways.com/travel...t/public/en_gb
If you do go ahead with BA (and assuming initially you end up in WT both ways) then once you've actually booked and got a booking reference you might find there's the option of an upgrade on an individual leg ( ba.com then "manage my booking"), but it's a "might", not a "definite".
Hope you get something sorted out.