I would still try and amend that booking, online or by phone, to use the N744100 code.

Like I said, the desk agent you ask to add the driver won't know the free driver rule. IME "Additional Driver will not be charged for Executive Club members" is a more customer friendly, but inaccurate way of saying "Additional Driver will not be charged on booking made using the BAEC Blue (or Silver or Gold) AWD code".

You will also miss out on 500 miles if not booked correctly. The other way to do this is to join Avis Preferred which should speed things up when you collect. Ring them to link this to you booking and ensure at that time they have the right AWD. Avis Preferred will get you a further 500 miles.