It's not booking through BA that gives you the U.K. protection; it's booking the flight operated by BA, even if it's sold with an AA codeshare flight number.

Note that if you collect AAdvantage miles, you will earn based on the pre-tax fare paid if you book AA flight numbers. If you book BA flight numbers, your AAdvantage earnings will be based on distance flown and fare class.

If you are interested in booking through BA, even persons under 50 can join AARP as an "Associate Member," and still get the BA discount.