MFU

Been looking to book a flight to MRU and use some of my hard earned Tesco BA miles.
AIUI I can upgrade one leg of W to J for 12500 miles? When I look at the 'book and upgrade using miles' link on the BA site, it only lets me book e.g. a W return and then I can upgrade one or both of the legs.
However, I wanted to book W outbound (then upgrade) and Y inbound. Anybody know if that is possible? If it is, then maybe it can be done over the phone? Or booking the flight and then MFU later?
Thanks!

AIUI I can upgrade one leg of W to J for 12500 miles? When I look at the 'book and upgrade using miles' link on the BA site, it only lets me book e.g. a W return and then I can upgrade one or both of the legs.
However, I wanted to book W outbound (then upgrade) and Y inbound. Anybody know if that is possible? If it is, then maybe it can be done over the phone? Or booking the flight and then MFU later?
Thanks!