In the past it's been a well-known trick (at least on this forum) that bassinet seats show as available from T-3d but can't actually be booked in MMB - however a phone call to BAEC will secure them assuming one has advance seat reservation rights.

I just tried to do exactly this for 15A on a 777 flying on Tuesday - which from today showed as available - and I was told quite categorically that it cannot be booked until OLCI opens - and furthermore this has been true for fourteen years (the time the agent has been working for BA). Can anyone shed any light on this?