This makes me smile.

So long as it's all more or less the same, they won't care. I'm a foreigner living in Spain and there is no concept of middle name here. So your "name" is "George James" and first surname would be "Smith" and then you just leave second surname field blank.

But, believe me that Iberia staff have seen every permutation humanly imaginable of messing that up for non-Spanish people. It won't be an issue at all, though I would be sure to have a copy of your ESTA should it need a manual override.

Now remember that combine my avios between BA and IB only works if the names are exactly alike and BA doesn't allow for the second surname so all Spanish people have to call in and do it by phone. It can be annoying both ways.