Well, I thought I'd update this just in case anyone searched and found it. The Graco Step2 seat does fit in BA seats, even the domestic one. It's a bit bulky to get to your seat, but it was the easiest to install and uninstall because of the recline feature. I had no trouble at all. It had about an inch of clearance for the width in the transatlantic flight, and about 1/2 an inch for the domestic.

I am flying again this summer but I won't be taking that seat. It was great, but it's just so heavy. Now my daughter is bigger I'm going for a booster and hoping that will work. However, the recline feature was fantastic for our red-eye transatlantic flight. She slept all but an hour of it. Amazing.

So if anyone else was wondering - yes, a Graco Step2 carseat does fit in BA seats