The OP will have unlimited joker access hence the deep interest in the D bucket.

The simple reason here is that D is a reasonably low margin bucket that you use to fill an aircraft. Right now the chance of Tokyo opening up for full travel is extremely low, so BA would be shooting itself in the foot to create a lot of contractural liability at this price point. If Japan does open up then the aircraft will sure be filled up at higher margin buckets and at that point BA can assess whether it joins in the market place. This may be one of the unintentional impacts of EC261.