Originally Posted by Gardyloo
... BAEC's MFU upgrades from (leisure, hence paid) Y to a comfier seat might make more sense.
True; it is also important to note that MFU's on BA go only one class higher. So from paid Y (World Traveler) you can to go W or T (World Traveler Plus, aka premium economy). If you paid for W or T (WT+), you can upgrade Club World. The best value are the WT+ to CW MFUs.