Expect the worst and they will surprise you!
It sucks flying with kids. We just did a UK round trip with a 2.5 year old and a 6 month old and it was hard work, we were tired as hell, but the girls were great all things considered.
Your husband sounds like mine. He may be able to fit like one toy and diaper in his bag, because he has to bring 17 gadgets. Grrr. Since you have the extra seat, keep your bag underneath that one, so that your stuff is accessible during takeoff/landing. Bring a change of shirt for you. I always manage to come out looking as if I purposely smeared food onto my shirt.
Re: seats. If you haven't called BA, it might be worth a shot to plead your case. If not just keep asking when you get to the airport.
One last tip: Pack or pick up something for you to eat with one hand. We were really able to eat the meals on this last flight because at least one of us always had a child in our lap. Oh and order a kids meal for the extra seat. It comes with some pretty tasty snacks.