If you book flight and car together on the Iberia website it will be booked through BA Holidays anyway. There is no real difference to booking via the BA website except it is often cheaper. If your flight has an IB flight number, you will not be able to select seats via the IB booking. But add your BAEC number when you book and within a few days you will have the booking show up in your BAEC app / account and will then be able to select seats in the normal way. Only other difference is that if you need to contact them after you book, you have to call an Iberia number (UK).