I would also agree that AA are not likely to find issue with this.

BAA at LHR (through which you are transiting) is another story. My wife has issues with a bag that was about 1/2" too big for the sizer. This was at LGW. She had to check it, and BA proceeded to lose it for 4 days...returning it the night we were leaving AMS.

On the way back, through LHR, they tried it again at FCC. Wouldn't budge. This time we cleared customs and came back through the Fast Track security line with no problems.

Mixed results for different people, but definitely a higher probability in BAA airports than in the US, IME. She now has a compliant bag

Cheers.