Good luck! The BA bassinet seats should take an 11 month old, though it will be a snug fit. The child can be strapped in, so that he can't get out by himself. I don't think there will be a problem with changing non-poo nappies at seat. As far as drinks go (water / milk) I don't know what you will be allowed to bring on board, you should ask that question of BA directly - or someone more in the know on here may answer. As far as you or your husband holding both kids - the question is more how long you can tolerate it for - but I imagine you have short periods in mind. You will have to have the child on your lap(s) with lap-belt on during take off and landing (and perhaps as well if the seat belt light come on during flight - don't know for certain). The whole thing sounds fairly hellish - have you considered phenergan?
Also, don't expect much help from BA staff on board - sometimes they're great, but I've experienced it when they're not - seemed like they could hardly have cared less - but I don't think this is necessarily their fault (company rules?). You need to make very sure that you have the bassinet row seats booked - I've also experienced it when pre-requesting the bassinet seat that on turning up at the airport (3 hours early) it has already been allocated to someone else
. This can happen particularly if there's more babies than bassinets on board.