You will not earn AA miles on the BOS-LHR and return legs, so your options are either to split the earning as you suggested, or take all of the credit on BA.
You will also not be able to upgrade any of the BA segments with AA miles, regardless of whether they are transatlantic or not. While BA is eligible (either through a partner award or through a oneworld award) for award tickets on routes that are not between the US and UK, those are not upgrade awards.
Mike