Did you book at ba.com using regular BA Prices (ie not industry rates?).
You can upgrade any WT+ fare to NCW for 25k BA Miles round trip subject to availability; if you need some extra BA Miles, just apply for the BA Visa at
www.ba.com and get 15k BA miles on first purchase for free. You can purchase the remaining 10k miles.
Much cheaper than paying for it. Tell us which flight and we can check if there is availability in Club for you.
ONe way upgrades for 12.5k BA Miles are only possible where the return sector has no upgrade availability in Club.